The presence of the Moon moderates Earth’s wobble on its axis, leading to a relatively stable climate over billions of years. From Earth, we always see the same face of the Moon because the Moon rotates once on its own axis in the same time that it travels once around Earth (called synchronous rotation).
The light areas of the Moon are known as the highlands. The dark features, called maria (Latin for seas), are impact basins that were filled with lava between 4 and 2.5 billion years ago.
Though the Moon has no internally generated magnetic field, areas of magnetism are preserved in the lunar crust, but how this occurred is a mystery. The early Moon appears not to have had the right conditions to develop an internal dynamo, the mechanism for global magnetic fields for the terrestrial planets.
How did the Moon come to be? The leading theory is that a Mars-sized body collided with Earth approximately 4.5 billion years ago, and the resulting debris from both Earth and the impactor accumulated to form our natural satellite. The newly formed Moon was in a molten state. Within about 100 million years, most of the global “magma ocean” had crystallized, with less dense rocks floating upward and eventually forming the lunar crust.
2013 Phases of the Moon
NOTE: All times are Universal time (UTC); to convert to local time add or subtract the difference between your time zone and UTC, remembering to include any additional offset due to summer time for dates when it is in effect.
| New Moon | First Quarter | |||
| Day | Time | Solar Eclipse |
Day | Time |
| 11/01/13 | 19:44:00 | 18/01/13 | 23:45:00 | |
| 10/02/13 | 07:20:00 | 17/02/13 | 20:31:00 | |
| 11/03/13 | 19:51:00 | 19/03/13 | 17:27:00 | |
| 10/04/13 | 09:35:00 | 18/04/13 | 12:31:00 | |
| 10/05/13 | 00:28:00 | Annular | 18/05/13 | 04:34:00 |
| 08/06/13 | 15:56:00 | 16/06/13 | 17:24:00 | |
| 08/07/13 | 07:14:00 | 16/07/13 | 17:24:00 | |
| 06/08/13 | 21:51:00 | 14/08/13 | 10:56:00 | |
| 05/09/13 | 11:36:00 | 12/09/13 | 17:08:00 | |
| 05/10/13 | 00:34:00 | 11/10/13 | 23:02:00 | |
| 03/11/13 | 12:50:00 | Hybrid Solar |
10/11/13 | 05:57:00 |
| 03/12/13 | 00:22:00 | 09/12/13 | 15:12:00 | |
| Full Moon | Last Quarter | |||
| Day | Time | Lunar Eclipse |
Day | Time |
| 05/01/13 | 03:58:00 | |||
| 27/01/13 | 04:38:00 | 03/02/13 | 13:56:00 | |
| 25/02/13 | 20:26:00 | 04/03/13 | 21:53:00 | |
| 27/03/13 | 09:27:00 | 03/04/13 | 04:37:00 | |
| 25/04/13 | 19:57:00 | Partial (Umbral) |
02/05/13 | 11:14:00 |
| 25/05/13 | 04:25:00 | Penumbral | 31/05/13 | 18:58:00 |
| 23/06/13 | 11:32:00 | 30/06/13 | 04:53:00 | |
| 22/07/13 | 18:15:00 | 29/07/13 | 17:43:00 | |
| 21/08/13 | 01:45:00 | 28/08/13 | 09:35:00 | |
| 19/09/13 | 11:13:00 | 27/09/13 | 03:55:00 | |
| 18/10/13 | 23:38:00 | Penumbral | 26/10/13 | 23:40:00 |
| 17/11/13 | 15:16:00 | 25/11/13 | 19:28:00 | |
| 17/12/13 | 09:28:00 | 25/12/13 | 13:48:00 | |
A waxing crescent moon 17 jan 2013
Perigee and Apogee Dates and Times
| Perigee | ||||
| Day | Time | Distance in kilometres a |
Closest or most distant b |
Interval c |
| 10/01/13 | 10:27:00 | 360047 | N-1d 9h | |
| 07/02/13 | 12:10:00 | 365313 | N-2d19h | |
| 05/03/13 | 23:21:00 | 369953 | N-5d20h | |
| 31/03/13 | 03:56:00 | 367493 | F+3d18h | |
| 27/04/13 | 19:49:00 | 362267 | F+1d23h | |
| 26/05/13 | 01:46:00 | 358374 | F+ 21h | |
| 23/06/13 | 11:11:00 | 356989 | ++ | F- 0h |
| 21/07/13 | 20:28:00 | 358401 | F- 21h | |
| 19/08/13 | 01:27:00 | 362264 | N-2d18h | |
| 15/09/13 | 16:35:00 | 367387 | N-3d18h | |
| 10/10/13 | 23:07:00 | 369811 | N+5d22h | |
| 06/11/13 | 09:29:00 | 365361 | N+2d20h | |
| 04/12/13 | 10:16:00 | 360063 | N+1d 9h |
| Apogee | ||||
| Day | Time | Distance in kilometres a |
Closest or most distant b |
Interval c |
| 22/01/13 | 10:53:00 | 405311 | F-1d17h | |
| 19/02/13 | 06:31:00 | 404473 | F-6d13h | |
| 19/03/13 | 03:14:00 | 404261 | N+7d 7h | |
| 15/04/13 | 22:23:00 | 404864 | N+5d12h | |
| 13/05/13 | 13:32:00 | 405826 | N+3d13h | |
| 09/06/13 | 21:41:00 | 406486 | – | N+1d 5h |
| 07/07/13 | 00:37:00 | 406491 | — | N-1d 6h |
| 03/08/13 | 08:54:00 | 405833 | N-3d12h | |
| 30/08/13 | 23:47:00 | 404882 | F-5d11h | |
| 27/09/13 | 18:18:00 | 404308 | F-7d 6h | |
| 25/10/13 | 14:26:00 | 404560 | F+6d18h | |
| 22/11/13 | 09:51:00 | 405445 | F+4d18h | |
| 19/12/13 | 23:50:00 | 406267 | + | F+2d14h |
a: For each perigee and apogee the distance in kilometres between the centres of the Earth and Moon is given. Perigee and apogee distances are usually accurate to within a few kilometres compared to values calculated with the definitive ELP 2000-82 theory of the lunar orbit; the maximum error over the years 1977 through 2022 is 12km in perigee distance and 6km at apogee.
b: The closest perigee and most distant apogee of the year are marked with “++” if closer in time to full Moon or “–” if closer to new Moon. Other close-to-maximum apogees and perigees are flagged with a single character, again indicating the nearer phase. Following the flags is the interval between the moment of perigee or apogee and the closest new or full phase; extrema cluster on the shorter intervals, with a smaller bias toward months surrounding the Earth’s perihelion in early January.
c: “F” indicates the perigee or apogee is closer to full Moon, and “N” that new Moon is closer. The sign indicates whether the perigee or apogee is before (“-“) or after (“+”) the indicated phase, followed by the interval in days and hours. Scan for plus signs to find “photo opportunities” where the Moon is full close to apogee and perigee
Moon phases
As the relative position of the Sun, Moon and Earth changes, differing proportions of the Moon’s visible surface are illuminated by the Sun. The phases of the Moon are specific instances in this process.
New moon
A new Moon occurs when the apparent longitudes of the Moon and Sun differ by 0°. At this time, the Moon does not appear to be illuminated.
First quarter
Occurs when the apparent longitudes of the Moon and Sun differ by 90°. At this time 50 per cent of the Moon’s visible surface is illuminated.
Full moon
Occurs when the apparent longitudes of the Moon and Sun differ by 180°. At this time 100 per cent of the Moon’s visible surface is illuminated.
Last quarter
Occurs when the apparent longitudes of the Moon and Sun differ by 270°. At this time 50 per cent of the Moon’s visible surface is illuminated.
Moonrise and moonset
Moonrise
Moonrise is defined as the instant when, in the eastern sky, under ideal meteorological conditions, with standard refraction of the Moon’s rays, the upper edge of the Moon’s disk is coincident with an ideal horizon.
Moonset
Moonset is defined as the instant when, in the western sky, under ideal meteorological conditions, with standard refraction of the Moon’s rays, the upper edge of the Moon’s disk is coincident with an ideal horizon.
Equinoxes and Solstices
The equinoxes represents either of two times of the year when the Sun crosses the plane of the Earth’s equator and day and night are of equal length, while the solstices is either of the two times of the year when the Sun is at its greatest distance from the celestial equator.
Uploaded on 20 Feb 2012
New images acquired by NASA’s Lunar Reconnaissance Orbiter (LRO) spacecraft show that the moon’s crust is being slightly stretched, forming small valleys – at least in some small areas. High-resolution images obtained by the Lunar Reconnaissance Orbiter Camera (LROC) provide evidence that these valleys are very young, suggesting the moon has experienced relatively recent geologic activity.
Smithsonian Institution Senior Scientist Tom Watters explains more about the moon’s recent geological activity in this short video.
Night Sky: Visible Planets, Moon Phases & Events, January 2013
How Moon Phases Work
via Space Dot Com
Fact or fiction?
The phases of the moon are caused by the shadow of the Earth falling on the moon.
Fiction!
This is probably the most commonly held misconception in all astronomy. Here’s how the moon’s phases really come about:
The moon is a sphere that travels once around the Earth every 29.5 days. As it does so, it is illuminated from varying angles by the sun. At “new moon,” the moon is between the Earth and sun, so that the side of the moon facing towards us receives no direct sunlight, and is lit only by dim sunlight reflected from the Earth. As it moves around the Earth, the side we can see gradually becomes more illuminated by direct sunlight.
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Here’s how the moon changes phases as it orbits the Earth, constantly changing the angle that sunlight hits the moon and is reflected, or not, to our eyes.
CREDIT: Starry Night Software
After a week, the moon is 90 degrees away from the sun in the sky and is half illuminated, what we call “first quarter” because it is about a quarter of the way around the Earth.
A week after this, the moon is 180 degrees away from the sun, so that sun, Earth and moon form a line. The moon is fully illuminated by the sun, so this is called “full moon.” This is the only time in the whole month when the Earth’s shadow is anywhere close to the moon. The Earth’s shadow points towards the moon at this time, but usually the moon passes above or below the shadow and no eclipse occurs.
A week later the moon has moved another quarter of the way around the Earth, to the third quarter position. The sun’s light is now shining on the other half of the visible face of the moon.
Finally, a week later, the moon is back to its new moon starting position. Usually it passes above or below the sun, but occasionally it passes right in front of the sun, and we get an eclipse of the sun.
So, the moon’s phases are not caused by the shadow of the Earth falling on the moon. In fact the shadow of the Earth falls on the moon only twice a year, when there are lunar eclipses.
This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions. Amateur astronomer Geoff Gaherty operates his own Foxmead Observatory in Coldwater, Ontario, Canada.
- Links to story at SEN: Our Solar System
| Moon Phase and Liberation, 2013 | ||||||||||||||||||
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Dial-A-Moon At NASA, new interactive tool.
The animation archived on this page shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year 2013, at hourly intervals. Until the end of 2013, the initial Dial-A-Moon image will be the frame from this animation for the current hour. Published on 20 Nov 2012 This visualization shows the moon’s phase and liberation throughout the year 2013, at hourly intervals. Each frame represents one hour. In addition, this visualization also shows other relevant information, including moon orbit position, sub earth and sub solar points, distance from the Earth. Click each graphic to learn more about what it means! Finally, to learn more about this visualization, or to see what the moon will look like at any hour in 2013, visit http://svs.gsfc.nasa.gov/goto?4000! This video is public domain and can be downloaded at:http://svs.gsfc.nasa.gov/goto?4000 The jagged, cratered, airless lunar terrain casts sharp shadows that clearly outline the Moon’s surface features for observers on Earth. This is especially true near the terminator, the line between day and night, where surface features appear in high relief. Elevation measurements by the Lunar Orbiter Laser Altimeter (LOLA) aboard the Lunar Reconnaissance Orbiter (LRO) make it possible to simulate shadows on the Moon’s surface with unprecedented accuracy and detail. The Moon always keeps the same face to us, but not exactly the same face. Because of the tilt and shape of its orbit, we see the Moon from slightly different angles over the course of a month. When a month is compressed into 24 seconds, as it is in this animation, our changing view of the Moon makes it look like it’s wobbling. This wobble is calledlibration. The word comes from the Latin for “balance scale” (as does the name of the zodiac constellation Libra) and refers to the way such a scale tips up and down on alternating sides. The sub-Earth point gives the amount of libration in longitude and latitude. The sub-Earth point is also the apparent center of the Moon’s disk and the location on the Moon where the Earth is directly overhead. The Moon is subject to other motions as well. It appears to roll back and forth around the sub-Earth point. The roll angle is given by the position angle of the axis, which is the angle of the Moon’s north pole relative to celestial north. The Moon also approaches and recedes from us, appearing to grow and shrink. The two extremes, called perigee (near) and apogee (far), differ by more than 10%. The most noticed monthly variation in the Moon’s appearance is the cycle of phases, caused by the changing angle of the Sun as the Moon orbits the Earth. The cycle begins with the waxing (growing) crescent Moon visible in the west just after sunset. By first quarter, the Moon is high in the sky at sunset and sets around midnight. The full Moon rises at sunset and is high in the sky at midnight. The third quarter Moon is often surprisingly conspicuous in the daylit western sky long after sunrise. Celestial north is up in these images, corresponding to the view from the northern hemisphere. The descriptions of the print resolution stills also assume a northern hemisphere orientation. To adjust for southern hemisphere views, rotate the images 180 degrees, and substitute “north” for “south” in the descriptions. |
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The phase and libration of the Moon for 2013, at hourly intervals. Includes supplemental graphics that display the Moon’s orbit, subsolar and sub-Earth points, and the Moon’s distance from Earth at true scale. Duration: 4.9 minutes Available formats: 1920×1080 MPEG-4 117 MB 1280×720 MPEG-4 57 MB 640×360 MPEG-4 20 MB 1920×1080 Frames (Fancy) 320×180 PNG 143 KB 160×80 PNG 40 KB 80×40 PNG 11 KB How to play our movies |
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The phase and libration of the Moon for 2013 at hourly intervals, with music, titles, supplemental graphics, and transcript. Duration: 5.3 minutes Available formats: 1920×1080 (29.97 fps) QT (YouTube) 285 MB 1280×720 (29.97 fps) QT (YouTube) 192 MB 1280×720 (29.97 fps) QT (ProRes) 2 GB 1280×720 (29.97 fps) WMV (Windows) 169 MB 960×540 (29.97 fps) MPEG-4 (AppleTV) 152 MB 640×360 (29.97 fps) QT 116 MB 640×360 (29.97 fps) MPEG-4 (iPod) 60 MB 320×240 (29.97 fps) MPEG-4 (iPod) 30 MB 320×180 PNG 143 KB How to play our movies |
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The phase and libration of the Moon for 2013, at hourly intervals. The full-resolution frames include an alpha channel. Duration: 4.9 minutes Available formats: 1920×1080 MPEG-4 84 MB 1280×720 MPEG-4 38 MB 640×360 MPEG-4 11 MB 1920×1080 Frames (Plain) 560×560 Frames 216×216 Frames 320×180 PNG 105 KB How to play our movies |
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The phase and libration of the Moon for 2013 at hourly intervals, with music, titles, and transcript. Duration: 5.3 minutes Available formats: 1920×1080 (29.97 fps) QT (YouTube) 169 MB 1280×720 (29.97 fps) QT (YouTube) 171 MB 1280×720 (29.97 fps) QT (ProRes) 1 GB 1280×720 (29.97 fps) WMV (Windows) 146 MB 960×540 (29.97 fps) MPEG-4 (AppleTV) 134 MB 640×360 (29.97 fps) MPEG-4 (iPod) 60 MB 320×240 (29.97 fps) MPEG-4 (iPod) 24 MB 640×360 (29.97 fps) QT 90 MB 320×180 PNG 105 KB How to play our movies |
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The orbit of the Moon in 2013, viewed from the north pole of the ecliptic, with the vernal equinox to the right. The sizes of the Earth and Moon are exaggerated by a factor of 30. The frames include an alpha channel. Duration: 4.9 minutes Available formats: 420×420 MPEG-4 10 MB 420×420 Frames 320×180 PNG 10 KB How to play our movies |
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| From this birdseye view, it’s somewhat easier to see that the phases of the Moon are an effect of the changing angles of the sun, Moon and Earth. The Moon is full when its orbit places it in the middle of the night side of the Earth. First and Third Quarter Moon occur when the Moon is along the day-night line on the Earth.The First Point of Aries is at the 3 o’clock position in the image. The sun is in this direction at the spring equinox. You can check this by freezing the animation at the 1:03 mark, or by freezing the full animation with the time stamp near March 20 at 11:00 UTC. This direction serves as the zero point for both ecliptic longitude and right ascension.The north pole of the Earth is tilted 23.5 degrees toward the 12 o’clock position at the top of the image. The tilt of the Earth is important for understanding why the north pole of the Moon seems to swing back and forth. In the full animation, watch both the orbit and the “gyroscope” Moon in the lower left. The widest swings happen when the Moon is at the 3 o’clock and 9 o’clock positions. When the Moon is at the 3 o’clock position, the ground we’re standing on is tilted to the left when we look at the Moon. At the 9 o’clock position, it’s tilted to the right. The tilt itself doesn’t change. We’re just turned around, looking in the opposite direction. | ||||||||||||||||||
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An animated diagram of the subsolar and sub-Earth points for 2013. The Moon’s north pole, equator, and meridian are indicated. The frames include an alpha channel. Duration: 4.9 minutes Available formats: 320×320 MPEG-4 5 MB 320×320 Frames 320×180 PNG 21 KB How to play our movies |
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| The subsolar and sub-Earth points are the locations on the Moon’s surface where the sun or the Earth are directly overhead, at the zenith. A line pointing straight up at one of these points will be pointing toward the sun or the Earth. The sub-Earth point is also the apparent center of the Moon’s disk as observed from the Earth.In the animation, the blue dot is the sub-Earth point, and the yellow dot is the subsolar point. The lunar latitude and longitude of the sub-Earth point is a measure of the Moon’s libration. For example, when the blue dot moves to the left of the meridian (the line at 0 degrees longitude), an extra bit of the Moon’s western limb is rotating into view, and when it moves above the equator, a bit of the far side beyond the north pole becomes visible.At any given time, half of the Moon is in sunlight, and the subsolar point is in the center of the lit half. Full Moon occurs when the subsolar point is near the center of the Moon’s disk. When the subsolar point is somewhere on the far side of the Moon, observers on Earth see a crescent phase. | ||||||||||||||||||
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An animated diagram of the Moon’s distance from the Earth for 2013. The sizes and distances are true to scale, and the lighting and Earth-tilt are correct. The frames include an alpha channel. Duration: 4.9 minutes Available formats: 1920×1080 MPEG-4 2 MB 1280×720 MPEG-4 1 MB 640×360 MPEG-4 656 KB 1920×1080 Frames (Distance) 320×180 PNG 1 KB How to play our movies |
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| The Moon’s orbit around the Earth isn’t a perfect circle. The orbit is slightly elliptical, and because of that, the Moon’s distance from the Earth varies between 28 and 32 Earth diameters, or about 356,400 and 406,700 kilometers. In each orbit, the smallest distance is called perigee, from Greek words meaning “near earth,” while the greatest distance is called apogee. The Moon looks largest at perigee because that’s when it’s closest to us.The animation follows the imaginary line connecting the Earth and the Moon as it sweeps around the Moon’s orbit. From this vantage point, it’s easy to see the variation in the Moon’s distance. Both the distance and the sizes of the Earth and Moon are to scale in this view. In the full-resolution frames, the Earth is 50 pixels wide, the Moon is 14 pixels wide, and the distance between them is about 1500 pixels, on average.Note too that the Earth appears to go through phases just like the Moon does. For someone standing on the surface of the Moon, the sun and the stars rise and set, but the Earth doesn’t move in the sky. It goes through a monthly sequence of phases as the sun angle changes. The phases are the opposite of the Moon’s. During New Moon here, the Earth is full as viewed from the Moon. | ||||||||||||||||||
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Waxing crescent. Visible toward the southwest in early evening.Available formats: 3600 x 3600 TIFF 8 MB 320 x 320 PNG 280 KB |
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First quarter. Visible high in the southern sky in early evening.Available formats: 3600 x 3600 TIFF 9 MB 320 x 320 PNG 293 KB |
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Waxing gibbous. Visible to the southeast in early evening, up for most of the night.Available formats: 3600 x 3600 TIFF 12 MB 320 x 320 PNG 352 KB |
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Full Moon. Rises at sunset, high in the sky around midnight. Visible all night.Available formats: 3600 x 3600 TIFF 16 MB 320 x 320 PNG 398 KB |
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Waning gibbous. Rises after sunset, high in the sky after midnight, visible to the southwest after sunrise.Available formats: 3600 x 3600 TIFF 12 MB 320 x 320 PNG 358 KB |
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Third quarter. Rises around midnight, visible to the south after sunrise.Available formats: 3600 x 3600 TIFF 10 MB 320 x 320 PNG 317 KB |
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Waning crescent. Low to the east before sunrise.Available formats: 3600 x 3600 TIFF 7 MB 320 x 320 PNG 281 KB |
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New Moon. By the modern definition, New Moon occurs when the Moon and Sun are at the same geocentric ecliptic longitude. The part of the Moon facing us is completely in shadow then. Pictured here is the traditional New Moon, the earliest visible waxing crescent, which signals the start of a new month in many lunar and lunisolar calendars.Available formats: 3600 x 3600 TIFF 6 MB 320 x 320 PNG 261 KB |
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Published on 20 Nov 2012 This visualization shows the moon’s phase (Only no detail) and liberation throughout the year 2013, at hourly intervals. Each frame represents one hour Another cool link to an Interactive MOON Guide |
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Night Sky: Visible Planets, Moon Phases & Events, January 2013
Mark Thompson’s guide to the Moon
Via SEN
By Mark Thompson 24 August 2011
There can be few objects that have inspired both artists and scientists as much as the Moon. Perhaps surprisingly its appearance has barely changed in the thousands of years that mankind has walked the Earth and ancient civilisations enjoyed much the same view as the one we see today. During the Moon’s relentless orbit around the Earth it has witnessed civilisations come and go, entire species evolve and die out and even continents slowly shift. The one thing that has changed over all those years though is our understanding of it, and its still giving us plenty of surprises.
As natural planetary satellites go, the Moon is actually quite large with a diameter of 3,476km (2,155 miles) around the equator. It orbits the Earth at an average distance of 384,400km (238,000 miles) but this varies from its closest, or perigee at 362,570km (225,000 miles) to its most distant point, or apogee of 405,410km (251,000 miles). There are a couple of things people will always think of when you mention the Moon: craters and phases which can both be observed without a telescope.
The phases of the Moon are simple to understand and anyone who has looked at it over a series of nights will notice that it changes progressively night after night with a whole cycle taking about a month. In fact the word month has its origin in the word Moon relating to the approximate length of a full lunar phase cycle. To understand the phases its important to realise that we only see the Moon because it’s a sphere and reflects sunlight – turn the Sun off and the Moon would no longer be visible.
We see the phases change as the Moon orbits around the Earth and the angle between the Sun and Moon alters. During a full Moon, the Sun and Moon are opposite each other in the sky and we see the fully illuminated or daytime face, but at new Moon they are both in the same direction and we see the night time portion of the Moon. As it moves around the Earth, the angle between the Earth, Sun and Moon changes and we see varying amounts of the daytime/nighttime side.The line between the illuminated and un-illuminated faces is called the terminator and its down this line where the Sun is just rising or setting.
From an observational point of view, the surface features are much more prominent if observed when they are near the terminator. The low altitude of the Sun from that point means the shadows cast by the features are much longer making them stand out clearly against the lunar surface. The worst time to observe the Moon is when it’s full and the shadows are minimal.
The phases of the Moon are a little more complicated than I’ve just explained though because the orbit of the Moon around the Earth is very slightly tilted with respect to the Earth’s orbit around the Sun. If it wasn’t then every time we had a full Moon the Earth would block sunlight from reaching the Moon and we would see a lunar eclipse. Clearly we don’t have one every month and its because the Moon’s orbit is tilted that on most occasions the Moon is slightly above or below the Earth’s shadow.
Look at the Moon more closely and you will see dark grey patches, turn binoculars or even a telescope on it and some will turn into great plains while others turn into cavernous craters. The craters were created by meteoric impacts where pieces of space rock smashed into the lunar surface. We see evidence of this process throughout the Solar System even here on Earth. The larger plains, or mare as they are properly called, are the aftermath of much larger impacts that have cracked the lunar surface allowing molten lava to seep up through the mantle. The lava solidifies over time leaving the plains we see today. Before good quality telescopes it was thought these great plains were actually lunar seas.
Another effect of the Moon’s orbit around the Earth are the tides. Like the Earth, the Moon has a gravitational pull and as a result it pulls on the Earth producing a bulge. As the Earth spins once on its axis it ‘passes underneath’ the bulge which we then experience as a tide. There are actually two bulges, one pointing roughly toward the Moon, the other in the opposite direction. When a location passes under the bulge it’s seen as high tide, hence we see two every day.
This bulge is pretty crucial and is having a big impact on the Earth-Moon system. You would think that the bulge lies directly between the Earth and Moon, given that it’s the pull of the Moon’s gravity that causes it. It turns out that the rotation of the Earth drags the bulge a little ahead of the Moon in its orbit. As it lays ahead of the Moon, the extra ‘lump’ of material produces a little extra pull on the Moon causing it to accelerate in its orbit. If you accelerate an orbiting object, it moves into a higher orbit -in other words, it moves further away. Thanks to the Apollo astronauts who left a special mirror on the surface, we can now accurately measure its distance and have found that the Moon is moving away from the Earth at a rate of 3.8cm per year!
It’s not only the Earth that experiences the tides, the Moon too has tides, though to a much lesser degree. The gravitational pull from Earth acts to distort the Moon and produce a lunar tidal bulge toward the Earth. When the Moon first formed it was spinning much faster than it does today and its rotation displaced the tidal bulge from its alignment between the Earth and Moon. The Earth’s gravitational pull still acted upon this bulge causing a braking effect on the Moon’s rotation. Over many millions of years this tidal interaction caused the Moon to slow down so much that it now rotates once on its axis for every orbit around the Earth, every 29.5 days. It’s an effect called captured or synchronous rotation and its result is that we now only ever see one half of the Moon from Earth. In reality we see can see a little more than 50% but this is due to the Moon’s orbital properties allowing us to glance a little further around.
With the Moon moving away from Earth it would be reasonable to assume that at some point they were in the same place. It is believed that the Moon was in fact once part of the Earth. At the time the Earth formed, the Solar System was a war zone with large chunks of rock and proto-planets flying around at ballistic speed. One piece about the size of Mars is thought to have smashed into the Earth throwing vast amounts of material into orbit. It’s believed that most of the heavy elements settled back on Earth while the lighter material stayed in orbit. Recent studies suggest that two moons could have formed, sharing the same orbit, which ultimately collided forming the Moon we see today. This new theory nicely accounts for the observation that one side of the Moon seems to have a much thicker crust which is now thought to be the remains of the Moon’s ancient companion.
Perhaps one of the most incredible discoveries in recent years was the discovery of water ice in some of the deep lunar craters. In these deep craters, that remain almost permanently in shadow, temperatures remain sub zero all year round allowing the ice crystals to form. This discovery opens up tantalising possibilities for future space exploration. The water molecules on the Moon could be harnessed for and purified for future explorers to drink. Taking this a step further, separate the water molecules into their hydrogen and oxygen components and they could be used to create rocket fuel for further onward exploration. No longer can we consider the Moon as a lifeless and hostile place, instead its becoming more likely that mankind’s next step out into the Solar System will involve using the Moon as an outpost for future giant leaps!
RELATED LINKS AT SEN and NASA
- NASA image gallery
- NASA fact file of the Moon
- The Sun
- Earth. That’s home, that’s us.
- Mercury
- Venus
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
- Guide to the solar system
- New clues to impact that created the Moon
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- Lunar Eclipses and Future Events (astroataglance.wordpress.com)
- NASA Considers Plan To Drag An Asteroid Into Moon’s Orbit (newscientist.com)
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Astro At A Glance 
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