I2AO Part 7: Earth, Sun and Moon
PART 7: EARTH, SUN and MOON by Lesa Moore
In this part, there are recommendations to download and view some online movies in various formats. If you are unable to play any of the movies, VLC media player is recommended for both Windows and Apple platforms. The free VLC download is available here.
- Relative sizes of the Sun, Earth and Moon
- The Sun
- The Sun's Atmosphere and its Features
- The Solar Spectrum
- Life Cycle - Formation of the Sun
- Life Cycle - The Main Sequence and the P-P Chain
- Life Cycle - Evolution, Old Age and the H-R Diagram
- Safely Observing the Sun
- The Moon
- The Earth
- Features of the Earth
- The Day, the Year, Solstices and Equinoxes
- Precession of the Equinoxes
- Lunar Eclipses
- Solar Eclipses
- Further Resources - YouTube video links and a Moon-phase worksheet
- The Sun's diameter is 1,392,000 km. The Sun contains 99.86% of the mass of the Solar System.
- The Earth's diameter is 12,756 km.
- The Moon's diameter is 3,476 km.
Figure 1, below, shows the relative sizes of the Sun and Earth (Earth is the small dark speck in the upper left). The Sun's diameter is 110x Earth's diameter. Figure 2, below, shows the relative sizes of the Earth (blue) and the Moon (grey).
Figure 1 - Sizes: Sun and Earth
Image Credit: Diagram by Lesa Moore
Figure 2 - Sizes: Earth and Moon
Image Credit: Diagram by Lesa Moore
- The Sun is a star, like any other star. It appears different from other stars simply because we're seeing it close-up.
- The Sun produces energy (heat and light) by nuclear fusion.
- The Sun is a big, hot ball of gas and is made predominantly from hydrogen and helium.
- The Sun's "atmosphere" has three layers - the photosphere (visible surface), the chromosphere (which lies just above the photosphere) and the corona (a deep, diffuse layer that is only seen during a total solar eclipse).
- The rotation of the Sun (25 days 9 hrs mean sidereal) was first identified by watching the changing positions of sunspots. The sunspot cycle is 11 years. In the cycle, the frequency, number and latitue of sunspots rises gradually, then falls more rapidly. The magnetic cycle is 22 years - each time the sunspot cycle resets, the north and south poles of the Sun flip, producing the 22-year magnetic cycle.
Figures 3 to 7, below, illustrate some features of the layers in the Sun's atmosphere.
Figure 3 below - The Photosphere: This view of the Sun shows the silhouette of Venus against the photosphere during a rare Transit of Venus across the face of the Sun. The photosphere is the "visible" surface of the Sun which emits black-body radiation (a continuous spectrum of all colours of light, including the rainbow colours and the invisible wavelengths from radio to gamma rays). The Sun is classified as a G2 yellow star with a surface temperature of 5800 K (or about 6000 C).
Image Credit: Catherine Braiding
Figure 4 below - Sunspots: Groups of sunspots can emerge and dissipate over a matter of days. This view of sunspots on the Sun was from 24 January 2013.
Image Credit: NASA/Goddard Space Flight Center, SDO/HMI instrument
Figure 5 below - Granulation around a Sunspot: This image shows the main sunspot in active region AR 12585 observed with The Swedish 1-m Solar Telescope (SST) on La Palma (Spain). Granulation is caused by convection cells in the photosphere. Sunspots are regions with strong magnetic fields that suppress convection. The central dark area, the umbra, has the strongest and most vertical magnetic fields. The dark umbra is surrounded by the filamentary penumbra where magnetic fields are more horizontal. Note that the black line in the lower left measures 2500 km on the Sun.
Instrument: Swedish 1-m Solar Telescope / CHROMIS wideband (wavelength 395.0 nm)
Center coordinates: (x,y) = (-113",23"), 5-Sep-2016
Observation: Luc Rouppe van der Voort and Shahin Jafarzadeh (University of Oslo, Norway)
Data reduction: Jaime de la Cruz Rodríguez (ISP/Stockholm University, Sweden)
Figure 6 below - The Chromosphere: The chromosphere of the Sun is most readily seen using a filter that only transmits light of a certain wavelength given off by hydrogen or helium. The chromosphere may feature prominences (pillars or loops of gas rising from the chromosphere), silhoutted against the background sky, as well as other types of active regions.
Image Credit: From NASA: This photograph of the Sun, taken on December 19, 1973, during the third and final manned Skylab mission, shows one of the most spectacular solar prominences ever recorded, spanning more than 588,000 kilometers (365,000 miles) across the solar surface. The loop prominence gives the distinct impression of a twisted sheet of gas in the process of unwinding itself. In this photograph, the solar poles are distinguished by a relative absence of supergranulation network, and a much darker tone than the central portions of the disk. Several active regions are seen on the eastern side of the disk. The photograph was taken in the light of ionized helium by the extreme ultraviolet spectroheliograph instrument of the U.S. Naval Research Laboratory. Source https://archive.org/details/S74-23458
Figure 7 below - The Corona: Visible during the 2012 total solar eclipse viewed from the hinterland of Cairns, this image shows the brilliant corona of the Sun. The black circle in the middle of the corona in the photo is the Moon, blocking out all the light from the photosphere and the chromosphere. During totality, for those standing in the shadow of the Moon, it is as dark as night. The corona is far hotter than the photosphere and, by itself, is about as bright as the full Moon.
Image Credit: Photo by Lesa Moore
Gases in the chromosphere and corona absorb light at discrete wavelengths in the continuous black-body spectrum emitted by the photosphere. The absorption leaves dark lines and bands in the spectrum that are used to identify the elements in the Sun’s atmosphere.
Figure 8 below - The Sun's Spectrum: A high resolution version of the spectrum of our Sun, this image was created from a digital atlas observed with the Fourier Transform Spectrometer at the McMath-Pierce Solar Facility at the National Solar Observatory on Kitt Peak, near Tucson, Arizona.
Image Credit: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
- A star forms from a cloud of gas (and dust) in space.
- The cloud collapses under gravity, making the core hot and dense.
- If the core becomes hot and dense enough to sustain nuclear fusion (at least 10 million K), a star is born.
- The size of the star depends on how much gas and dust was in the original cloud.
- Some material from the cloud may collapse into a disk around the star and, later, form planets. Hence, such a disk is termed a proto-planetary disk, or "proplyd" (refer Figure 9, below).
- The Sun is a middle-sized star that formed from a middle-sized cloud.
- The Sun formed about 4.5 billion years ago and is about half-way through its lifetime.
Figure 9 below - Stars forming in the Orion Nebula: A Hubble Space Telescope view of a small portion of the Orion Nebula, captured by the Wide Field and Planetary Camera 2, reveals five young stars. Four of the stars are surrounded by gas and dust trapped as the stars formed, but were left in orbit about the star. These are possibly protoplanetary disks, or proplyds, that might evolve on to agglomerate planets. The proplyds which are closest to the hottest stars of the parent star cluster are seen as bright objects, while the object farthest from the hottest stars is seen as a dark object. The field of view is only 0.14 light-years across. The Orion Nebula star-birth region is 1500 light-years away, in the direction of the constellation Orion the Hunter. Imaged 29 December 1993.
Image Credit: NASA; Caption author - C.R. O'Dell/Rice University; NASA
- Stars spend most of their lives on the Main Sequence, converting hydrogen to helium.
- The main reaction occurring in the Sun (and in stars on the Main Sequence) is the proton-proton (p-p) chain, which produces a helium (He) nucleus from four hydrogen (H) nuclei (refer Figure 10 below).
- A hydrogen nucleus is simply a proton.
- The mass of the He nucleus is less than the mass of four protons – the difference is released mainly as energy, plus some very low-mass particles.
- The total energy released is consistent with Einstein's equation, E = mc2.
- Energy released exerts outward pressure from the core. Gravity tries to pull the gas inwards. A stable star, like the Sun, is in balance. This balance is known as hydrostatic equilibrium.
- The core of the Sun is 10x as dense as lead.
- 1038 p-p chain conversions occur each second.
- The Sun converts 4 million tons of H to energy every second.
- It takes a million years for the energy to reach the surface (and then eight minutes to reach Earth).
Figure 10 below - The Proton-Proton Chain: In this chain reaction, particles collide under tremendous pressure and at very high temperature to overcome electrostatic repulsion. The collision of two protons forms a deuteron (converting one proton to a neutron). The collision of a proton and a deuteron forms helium-3 (two protons and one neutron). When two helium-3 nuclei collide, they produce helium-4 (two protons and two neutrons). Energy is released in the form of a positron (the anti-matter version of an electron), a neutrino (an extremely low-mass particle) and a gamma ray. Two protons are emitted in the final step, propagating the nuclear chain-reaction.
Image Credit: Diagram by Lesa Moore
- Stars go through a life cycle, from formation, through a long period on the Main Sequence, and then through different final stages depending on the size of the star. The Hertzsprung-Russell Diagram (or H-R Diagram, refer Figure 11 below) is a chart of properties of stars, with small stars at the bottom and large ones at the top, hot stars at the left and cool stars at the right. Unlike artist's colours, the hottest stars are blue and the coolest are red. The colours and temperatures refer to properties of the stars' photospheres.
- In the H-R Diagram, the majority of stars form the diagonal band of the Main Sequence. Smaller stars on the Main Sequence are the cooler, redder ones at lower right. The largest, brightest and hottest are at the upper left of the diagram. The Sun is about in the middle.
- When the Sun starts to run out of hydrogen fuel in its core, the reactions holding it up against gravity will slow down and it will shrink. This will boost the core temperature and will most likely ignite helium fusion in the core, while regenerating hyrogen fusion in the surrounding shell. These new fusion zones will cause the Sun to swell up and become a red giant. The expanded, diffuse atmosphere will extend much further away from the core, making the "surface" temperature much cooler, therefore redder in appearance.
- The red giant stars form the upper right branch on the H-R Diagram.
- Subsequent to helium fusing to carbon and oxygen, the Sun may go on to fuse carbon and oxygen to neon, but the fusion processes will eventually run out of fuel and heat. The Sun will not go supernova.
- At some point during the red giant stage, the Sun's atmosphere will likely encompass the orbits of Mercury and Venus, and the surface of the Earth will become too hot to retain any liquid water.
- When the fuel runs out, the outer atmosphere of the Sun will waft off into space and the core will collapse to become a white dwarf star, about the size of the Earth. The surrounding gas will become a planetary nebula (refer Figure 12 below).
- The whole life-cycle of the Sun is illustrated in Figure 13 below.
Figure 11 below - The Hertzsprung-Russell Diagram: This chart of stellar properties is used to understand how stars evolve by looking at a great population of stars. Hot stars are on the left, cool stars are on the right. Bright (large) stars are at the top, small (faint) stars are at the bottom. The Sun is roughly in the middle. Stars all along the Main Sequence are fusing hydrogen into helium in their cores. The red giants, to the upper right of the diagram, are going through later stages of fusion, producing the heavier elements beyond helium.
Image Credit: European Space Agency (ESA). This H-R Diagram is constructed using data from the Hipparcos satellite. Hipparcos is an acronym for HIgh Precision PARallax COllecting Satellite. Appropriately, the proununciation is also very close to Hipparchus, the name of a Greek astronomer who lived from 190 to 120 BC. By measuring the position of the Moon against the stars, Hipparchus was able to determine the Moon's parallax and, thus, its distance from the Earth. Similarly, Hipparcos measured parallaxes to get distances to stars. Accurate distances are necessary to determine stars' intrinsic brightnesses (or absolute magnitudes) so that they can be properly placed on the H-R Diagram.
Figure 12 below - Planetary Nebula Shapley 1: This planetary nebula surrounds the white dwarf star visible at the centre. The Sun will end up something like this in another five billion years.
Image Credit: Brent Miszalski
Figure 13 below - Summary Diagram of Evolution of the Sun: The Sun, from birth to finale as a white dwarf.
Image Credit: Prezi
- NEVER look directly at the Sun with the naked eye or any kind of optical device UNLESS you have a solar filter specifically designed for safe solar observing.
- Eyepiece filters sold with some telescopes are DANGEROUS and should NOT be used.
- Specially-marketed "eclipse shades" are suitable for use at naked-eye magnification (refer Figure 14, below). These are suitable for observing solar eclipses and the very largest of sunspots.
- Pinhole projection is another safe method of watching the progress of a solar eclipse, but is unlikely to show any detail on the photosphere.
- To see detail on the surface of the Sun, a telescope will be needed, but must be used either with a full-aperture solar filter over the front of the telescope (refer Figure 15, below), or in projection mode (refer Figure 16, below).
Figure 14 below - Eclipse Shades: Eclipse shades are safe for looking at the Sun at naked-eye magnification. This is adequate to observe a solar eclipse or a transit of Venus or Mercury across the face of the Sun.
Image Credit: Photo by Sarah (Chamberlain) Wood
Figure 15 below - A Full-Aperture Solar Filter: The solar filter on the front end of this telescope looks like a mirror. The only light visible through it is sunlight. For safety, the spotter scope remains capped.
Image Credit: Photo by Lesa Moore
Figure 16 below - Telescope Projection: The projection method is completely safe, once set up correctly. The observer views a projected image on a screen and never actually looks through the telescope. This method can also be used with one side of a pair of binoculars, when suitably baffled.
Image Credit: Photo by Geoff Sims
- The Moon has a cratered surface, which remains unweathered by anything except cosmic particles. The Moon has no atmosphere and no water.
- The Moon gives off no light of its own, but shines by reflected sunlight.
- The line dividing the sunlit part that we see from the dark part is known as the "terminator".
- The surface temperature of the Moon ranges between -180 C on the "night" side and +100 C on the "day" side.
- Mare (pronounced mah-ray, Latin for "seas") are the large dark areas which are composed of solidified basalt lava that escaped billions of years ago from a molten interior, when impactors struck the surface during the "late heavy bombardment" phase of the Solar System's history.
- The interior of the Moon is no longer molten.
- Bright rays from some craters are most easily seen around the time of full Moon. Each system of rays was formed by ejecta escaping through low points in the crater rim when the crater was formed by an impactor (refer Figure 17 below).
Figure 17 below - The 13-day-old Moon: A view of the Moon in the orientation that would be seen by a southern-hemisphere observer without a telescope. The phase is that seen 13 days after new Moon (or one day before full Moon). In this telescope view, note how the craters are easiest to see near the terminator (right-hand edge), where the sunlight is falling at a shallow angle. Rays are visible in the top-right area of the Moon's surface, centred on Tycho crater.
Image Credit: Uncredited. If it's yours, please let me know.
- The Moon is the brightest object in the night sky, but is not visible every night. It depends where the Moon is in its orbit around the Earth.
- The Moon orbits the Earth from west to east, taking it from new Moon, through first quarter to full Moon, then third quarter and, finally, back to new Moon.
- When it is less than half-lit, it is a crescent. When it is more than half-lit, the phase is called gibbous.
- In the first two weeks after new Moon, the phase is waxing (the bright part we see is increasing from night to night). After new Moon, as the sunlit amount we see is decreasing, the phase is waning.
- The phases of the Moon are caused by the simple geometry of the positions of the Sun and Moon relative to Earth (refer Figure 18 below).
- As the Moon orbits the Earth, it keeps the same side facing the Earth all the time. This is known as "captured rotation". There are video representations here: for example, download and run the video titled "morot1.avi" to see a sped-up simulation of the Moon orbiting the Earth.
- This may make you think that the Moon doesn't spin ... but it does! It spins once on its axis (relative to the stars) with each orbit around the Earth. To understand what would happen if the Moon didn't spin, and compare it with its synchronous rotation, download and view the movie "MOONSYNC.MOV" here.
- The sidereal period of the Moon's orbit (i.e. one orbit relative to the background stars) takes 27 days 7 hours.
- The Lunar (synodic) month is measured from new Moon to new Moon and takes a bit longer - 29 days 13 hours. This is because, during the time it has taken for the Moon to do one orbit of the Earth, the Earth has moved along in its orbit around the Sun, meaning that the Moon has had to go a bit more than a full orbit around the Earth to get back to the new Moon position in line with the Sun.
- The Moon's orbit around the Earth is elliptical. When the Moon's closest approach coincides with a full Moon, the media like to call it a "Supermoon".
Figure 18 below - Phases of the Moon: The inner part of the diagram shows the Earth in the centre and the Moon in various positions around the Earth with sunlight coming in from the left. The view is from the south and the orbit of the Moon is clockwise from this perspective. Each Moon image around the outside shows the corresponding phase of the Moon, as viewed from the southern hemisphere, as it would be seen in the sky during the evening, night or early morning. Note that, at certain phases, the Moon is also visible in the daytime (not illustrated).
Image Credit: Compiled by Lesa Moore, adapted from a NASA Moon phases diagram (https://solarsystem.nasa.gov/resources/676/phases-of-the-moon/) and a NASA south-pole image.
Take this link to view a video animation of the Moon's phases for a whole year (in two-and-a-half minutes). The animation archived on that page shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year 2011, at hourly intervals. There is further detail in the description on the YouTube web page.
- The main feature of Earth that makes it unique to the extent of current knowledge is life! To date, no life has been observed on any other planet or moon of the Solar System, or detected anywhere else in space.
- Earth has plate tectonics, including continental drift, folding, faulting, subduction and volcanoes. These systems occur because Earth has a combination of a solid rocky crust, a mantle of liquid rock, and oceans of liquid water. The water contributes to explosive volcanism.
- Earth has a permanent atmosphere composed of approximately 78% nitrogen, 21% oxygen 0.97% argon, 0.04% carbon dioxide, trace amounts of other gases, and 0.4% water vapor averaged over the entire atmosphere.
- Ozone in the atmosphere blocks harmful radation (gamma rays, x-rays and much of the UV) given off by the Sun.
- The permanent liquid water on the surface, combined with energy from sunlight, maintains the water cycle, i.e. evaporation, clouds, rain, snow, rivers, glaciers, lakes and oceans. The water cycle produces our weather and causes erosion.
Figure 19 below - Layers of Earth's Atmosphere: Diagrams like this give the impression that the atmosphere extends far out into space. In fact, 100km up is considered to be the boundary between the atmosphere and space, though the most diffuse portion of the atmosphere does continue upwards for several hundred kilometres. Compare this with the next Figure to get a truer perspective.
Image Credit: Diagram from the University Corporation for Atmospheric Research (UCAR) https://scied.ucar.edu/atmosphere-layers
Figure 20 below: Earth from Space: Thunderstorms on the Brazilian horizon are featured in this image photographed by an Expedition 20 crew member on the International Space Station. A picturesque line of thunderstorms and numerous circular cloud patterns filled the view as the station crew members looked out at the limb and atmosphere (blue line on the horizon) of Earth. Sunglint is visible on the waters of the Rio Madeira and Lago Acara in the Amazon Basin. Widespread smoke haze over the basin gives the reflected light an orange hue. The Rio Madeira flows northward and joins the Amazon River on its path to the Atlantic Ocean.
Image Credit: NASA's Image and Video Library https://images.nasa.gov/details-iss020e047807, NASA ID: iss020e047807, 6 October 2009
- Our day (and night) is caused by the spin of the Earth on its axis. One solar day is 24 hours (from noon one day to noon the next day, averaged over the year). One sidereal day (measured with respect to the background stars) is shorter - 23 hours 56 minutes. The solar day is longer because, as the Earth spins, it is also going around the Sun and has to spin a little bit further to bring the Sun overhead again from its new position one day later.
- Our year is the time taken for the Earth to orbit once around the Sun. The tropical year is approximately 365.25 day long. To manage the quarter-day, we have a system of leap years.
- Seasons result from the axial tilt of the Earth (23.5 degrees) with respect to its orbit around the Sun. While the tilt is maintained with respect to the background stars, the orbit moves the poles alternately into and out of sunlight, causing warming and cooling of Earth's hemispheres in seasonal cycles (refer Figure 21 below). A common misonception is that seasons are caused by our elliptical orbit moving the Earth closer to and futher away from the Sun. This variation, in fact, has virtually no effect on climate and would not account for the opposing seasons in the northern and southern hemispheres.
- Solstices occur when the Sun reaches the northern and southern limits of its apparent path through the sky as observed from Earth. At the northern hemisphere summer solstice (and the southern hemisphere winter solstice), the Sun is overhead at the Tropic of Cancer. The opposite solstices occur when the Sun is overhead at the Tropic of Capricorn. The latitude of the tropics (north and south) is the same as the axial tilt of the Earth - 23.5 degrees.
- Equinoxes occur when the Sun crosses the equator. Spring and autumn equinoxes correspond with the respective seasons. However, the "Vernal Equinox" is a special term that refers to the spring equinox for the northern hemisphere, or when the Sun's apparent path crosses Earth's equator from south to north.
- At the Vernal Equinox, the Sun's position in the sky, with respect to the background stars, is called the First Point of Aries. More on this below, under "Precession of the Equinoxes".
- Aurorae are caused by the interaction of protons and electrons from the Sun with the Earth's atmosphere. These particles, along with radiation, are part of the solar wind. The solar wind also drives back comet tails.
- Aurorae occur near the poles because Earth's magnetic field deflects charged particles, causing them to spiral along magnetic field lines, thus carrying them towards the poles where they descend and interact with the atmosphere.
- Names for the aurorae are: the northern lights, the southern lights, the Aurora Borealis (for the northern) and the Aurora Australis (for the southern).
- Auroral displays are more likely to occur around the time of solar maximum (i.e. the peak in the 11-year sunspot cycle).
- The colours of the aurora may be red, green or purple, depending on altitude (refer Figure 22 below). The highest aurorae are called "sprites" and "jellyfish" (refer Figure 23 below).
Figure 22 below - The Aurora Borealis: Mostly green, but with tinges of red at the upper extremes, this aurora was photographed from the deck of a cruise ship, about 9pm local time, and somewhere south of Tromsø (still well inside the Arctic Circle).
Image Credit: Mel and Penny Davis.
Details: tripod, Fujifilm X-T2 camera, 13-55mm Fujinon f/2.8 lens at 13mm, ISO 3200, one frame of a burst at 3 frames per second.
Figure 23 below - Sprites: The red features near the centre of the image are very high-altitude sprites. These ones are "jellyfish" sprites that typically occur between 50 and 90 km in altitude and measure up to 48 km across.
Image Credit: Greg Priestley captured this extraordinary image from a series of time-lapse images.
- Precession of the Equinoxes is due to an interaction between the Earth, the Sun and the Moon.
- Review Precession of the Equinoxes here.
Ocean tides on Earth are due to interactions between the Earth, the Sun and the Moon.
- Spring tides occur when the Sun, Moon and Earth are in line with each other. The name comes from the interpretation that the water is "springing" up between low and high tide.
- Neap tides are lesser tides that occur when the Sun and the Moon are at right-angles to each other, with respect to the Earth. Refer Figure 24, below, to see the difference between spring and neap tides.
- King tides occur around the time of year when the Earth is closest to the Sun in its elliptical orbit (perihelion, around 2 January). The highest tides occur when spring tides coincide with perihelion. Figure 25, below, shows exaggerated geometry to illustrate the conditions for a king tide.
- Some locations on Earth experience semi-diurnal tides (two highs and two lows a day), while some have diurnal tides (one high and one low per day). Figure 26, below, shows how the position of the Moon plays a role in the pattern of tides. Note that the Moon's orbit is inclined at an angle of five degrees to the ecliptic, so its declination (angle above or below the equator) can reach 28.5 degrees.
- A "dodge tide" is reported from South Australia, where the pattern of tides appears to skip one of its high tides (refer Figure 27 below). This is due to the particular shape, depth and gradient of the harbour at Adelaide Outer Harbour.
- One of the most dramatic tidal changes occurs at the Bay of Fundy. There are many time-lapse videos available online. One example is this one: https://www.youtube.com/watch?v=irSF8EOa034. Seeing this time-lapse, and keeping in mind that the Earth spins daily while the Moon takes a month to orbit, it seems reasonable to think of the continents as moving through the tidal bulges as the Earth spins, rather than the waters rising and falling at a single location.
Figure 24 - Spring and Neap Tides: The difference between spring and neap tides is due solely to the position of the Moon, which changes through the month as it orbits the Earth. The Sun and Moon both contribute to the tides but the Moon, though smaller, is much closer and contributes about two-thirds of the effect.
Image Credit: Diagrams by Lesa Moore
Figure 25 - King Tides: The largest tides occur when the Earth is closest to the Sun, i.e. when it is at perihelion around 2 January each year (right-hand side of diagram). M is the Moon, E is the Earth and S is the Sun. Diagram not to scale.
Image Credit: Diagram from “Our Restless Tides” by NOAA
Figure 26 - Diurnal and Semi-Diurnal Tides: Regions in high latitudes may experience diurnal (once daily) high and low tides due to the sometimes high declination of the Moon. Equatorial regions normally have semi-diurnal tides (two highs and two lows a day).
Image Credit: Diagram from "Our Restless Tides" by NOAA
Figure 27 - The Dodge Tide: This tide chart, from Adelaide Outer Harbour, shows larger and smaller high tides daily. They decrease until one of the high tides is skipped. The name was reportedly in use from 1938 and comes from the tide dodging its regular time to occur.
Image Credit: Australian Government Bureau of Meteorology (BOM)
- There are two types of eclipses that occur through Earth-Sun-Moon interactions. The ones where the Moon is darkened are called lunar eclipses. The ones where the Sun is blocked out are called solar eclipses.
- The geometry for a lunar eclipse is illustrated in Figure 28 below. The Moon moves through Earth's shadow, where that shadow projects out into space.
- There are two regions of the shadow. In the umbra, no sunlight is getting through. In the surrounding penumbra, the sunlight is only partly blocked.
- To have a total lunar eclipse, the Moon must move fully into the umbral shadow of the Earth.
- Lunar eclipses are easily observed. No special equipment is required and everyone on the night-time side of Earth during the time of the eclipse will be able to see it if the sky is clear.
- Lunar eclipses can last up to 100 minutes, including the partial phases before and after totality.
- During the partial phases, the Earth's shadow looks as though it takes a bite out of the Moon (refer Figure 29 below). It is quite different from the normal monthly lunar phases (compare, for example, with the gibbous phase in Figure 18 above).
- An eclipse of the Moon can only occur on a night of full Moon.
- During totality, the Moon does not go completely black. Light that has passed through Earth's atmosphere still reaches it. The light is reddened in the same way that a sunset is red, so this give a ruddy hue to the totally eclipsed Moon (refer Figures 29, 30 and 31 below).
Figure 28 below - Geometry of a Lunar Eclipse: If the Moon passes completely into the umbral shadow of the Earth, a total lunar eclipse is seen. If it is only partly in the umbral shadow, it will be a partial lunar eclipse. Sometimes, the Moon only goes into the penumbral shadow and experiences a penumbral eclipse, which is barely noticeable to the untrained eye.
Image Credit: Wikimedia Commons
Figure 29 below - Lunar Eclipse Photos: These images show stages of a lunar eclipse. In the over-exposed image on the left, a small "bite" has been taken out by the Earth's shadow. In the second image, the Moon has progressed further into the shadow. In the third image, a hint of red colouring may be seen on the shaded part of the Moon. At totality, in the right-hand image, the illumination is uneven because the Moon is not in the exact centre of the shadow.
Image Credit: Photos by Lesa Moore
Figure 30 below - A Lunar Perspective: The sequence of images at the bottom of this graphic give some idea of what a total lunar eclipse would look like from the Moon. In the diagrams above the images, the first is before the eclipse begins (corresponding with the first image), the second is a partial stage (corresponding with the second image), and the third diagram is totality, matching the fourth image. In that final image, the reason for the reddening of the Moon is evident - sunlight is refracted through Earth's atmosphere.
Image Credit: Wikimedia Commons
Figure 31 below - Total Lunar Eclipse Composite: These images, taken in a time sequence, have been compiled into a single image to illustrate the actual size of the Earth's shadow during this particular eclipse. Note how the Moon is not in the centre of the shadow, which can be imagined as a complete circle extending above this image. It may also be seen how the totally eclipsed Moon can be unevenly illuminated when it is not in the centre of the shadow.
Image Credit: Wang Letian
- Solar eclipses may be partial, total or annular, depending on the relative positions and separations of the Moon, Earth and Sun during the eclipse.
- A solar eclipse can only occur at the time of a new Moon, and must be observed in the daytime.
- The geometry for a solar eclipse is illustrated in Figure 32 below. The Moon passes between the Sun and the Earth, casting a shadow onto Earth's surface.
- As for a lunar eclipse, there are two regions of the shadow. In the umbra, no sunlight is getting through and the observer will see a total solar eclipse. In the surrounding penumbra, the sunlight is only partly blocked and the observer will see a partial solar eclipse.
- Solar eclipses will only be seen by observers located in the path of the shadow. The exact size of the shadow, umbral and penumbral regions, and duration of the eclipse depend on the exact distances between the Earth and the Moon and also between the Earth and the Sun (Earth's orbit around the Sun and the Moon's orbit around the Earth both being elliptical). Figure 33, below, shows the size of the shadow in an eclipse over Turkey, as viewed from the International Space Station.
- On some occasions, the Moon is too far away from Earth to completely cover the disk of the Sun. Although standing in the centre of the umbra, an observer will see an annular eclipse, where the Moon's disk sits inside the Sun's, leaving a ring or annulus of the photosphere visible. Figure 34, below, shows the stages of an annular eclipse.
- Totality, in a total solar eclipse, may last from a fraction of a second up to a maximum of seven-and-a-half minutes.
- During the partial phases, before and after totality, and for the entire time during an annular eclipse, either projection methods or appropriate solar filters must be used to avoid permanent damage to the eyes (eclipse shades for the eyes or full-aperture filters for telescopes and optical equipment). Figure 35, below, shows how simple pinholes in a piece of paper can be used to monitor the partial phases of a solar eclipse.
- During totality, the solar corona is visible, and the entire photosphere is covered by the Moon. Only then is it safe to observe the eclipse with unprotected eyes and equipment. Figure 36, below, is an image of the totally eclipsed Sun, surrounded by the corona.
- Other special apparitions that may be observed just before and after totality are the diamond-ring effect and Baily's Beads. Both are caused by the last vestiges of sunlight peeking through gaps in crater rims on the limb of the Moon. Both a "diamond ring" and a few smaller "beads" are seen in Figure 37 below.
Figure 32 below - Geometry of a Solar Eclipse: For a solar eclipse to occur, the Moon must pass exactly between the Sun and the Earth. The Moon's shadow traces a narrow path across the surface of the Earth and only those people in the path of totality will see the total solar eclipse. Either side of that, those in the penumbral shadow will observe a partial solar eclipse. For everyone outside the penumbral shadow, no eclipse will be visible.
Image Credit: Wikimedia Commons
Figure 33 below - Total Solar Eclipse viewed from Space: Astronauts on the International Space Station observed this total solar eclipse over the coast of Turkey. Cyprus is visible at top-centre of the image.
Image Credit: NASA Image and Video Library, https://images-assets.nasa.gov/image/iss012e21343/iss012e21343~orig.jpg
Figure 34 below - An Annular Eclipse: When the Moon is a bit too far away to fully cover the disk of the Sun, an annular eclipse occurs. In this series of images of an annular eclipse, the colour is caused by the solar filter used to protect the observer from eye damage. Note that prominences are visible on the limb of the eclipsed Sun.
Image Credit: Mel Davis
Figure 35 below - Pinhole Projection: All you need is a piece of paper and a pin to make small holes in any pattern you choose. Another piece of paper or any smooth surface may be used as a projection screen. A colander or any type of perforated sheet can also be used. Each hole projects a separate image of the Sun. Even sunlight passing through the leaves of trees onto the ground can produce the same effect.
Image Credit: Photos by Lesa and Peter Moore
Figure 36 below - Totality: During totality, the corona of the Sun becomes visible. This glow from the Sun's outermost atmosphere, about as bright as the full Moon, is normally completely overwhelmed by the light from the photosphere. During totality is the ONLY time that it is safe to observe the eclipse without eye protection.
Image Credit: Ross and Robyn
Figure 37 below - The Diamond Ring and Baily's Beads: Just before and after totality, it is possible to see sunlight passing through gaps in the mountains along the edge of the Moon. The best "diamond ring" is when there is one single shaft of sunlight. When there is a string of small beads of light, it is referred to as Baily's Beads. Both effects are seen in this image.
Image Credit: Mel Hulbert
Figure 38 below - Future Solar Eclipses: NASA has a website dedicated to eclipses here: https://eclipse.gsfc.nasa.gov/eclipse.html. The image below is from the world atlas of solar eclipse maps here: https://eclipse.gsfc.nasa.gov/SEatlas/SEatlas3/SEatlas2021.GIF.
Image Credit: NASA and Fred Espenak
To view a time-lapse video of a total solar eclipse, visit Geoff Sims' page on the Total Solar Eclipse of 14th November, 2012 (Pormpuraaw, QLD, Australia). Scroll to the bottom of the page for the time-lapse video: http://www.users.on.net/~simsg/astro/tse2012.htm.
- There are many video clips of solar prominences available online. One example is here: https://www.youtube.com/watch?v=NXzFgqQw6T8.
- NASA has a good explanation and footage of a coronal mass ejection from the Sun here (3-minute video): https://www.youtube.com/watch?v=sg3NAdOYp8Q
- Moon observing exercise (suitable for southern hemisphere observers): Get a feel for the Moon's orbit around the Earth by observing at the same time every evening for two weeks from new Moon. Fill in you observations on this template. Note the time of your first observation and observe at the same time every night. This brings your location on the surface of the Earth back to the same position with respect to the Moon's orbit each night so that the movement you measure is due to the orbit of the Moon, not the spin of the Earth. Hopefully, this will help you understand how the Moon orbits the Earth from west to east progressively through the month. The diagram is suitable for southern hemisphere observers viewing from west-north-west (right after new Moon), through north (at first quarter), to east (at full Moon). Download the PDF file here.
Author: Lesa Moore, 25th June 2020