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04/14/2022 – Ephemeris – The Moon appears to wobble
This is Bob Moler with Ephemeris for Thursday, April 14th. Today the Sun will be up for 13 hours and 26 minutes, setting at 8:26, and it will rise tomorrow at 6:58. The Moon, 2 days before full, will set at 6:43 tomorrow morning.
Tonight’s Moon is nearly full, only two days to go. In the evening, the little dark sea on the Moon called Mare Crisium or Sea of Crises is near the top of the Moon. Now it’s real close to the top edge of the Moon, what astronomers call the limb. Ten days ago, when the Moon was a crescent, Mare Crisium was farther from the limb. This wobbling motion over the lunar month is called libration. This occurs because the Moon has an elliptical orbit of the Earth and speeds up when it’s closer and slows when it’s farther away. However, the Moon’s rotation is constant due to its angular momentum. The period of the revolution of the Moon around the Earth exactly matches its rotation. When the Moon is farthest, its rotation gets a bit ahead of its revolution around the Earth. When closest, the rotation lags a bit. So it appears to wobble over the lunar month.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT – 4 hours). They may be different for your location.
Addendum
The Moon as it should appear at 10 pm tonight, April 14, 2022. Note the position, relative to the Moon’s limb, of the small lunar sea called Mare Crisium near the top of the Moon. Mare Crisium means Sea of Crises. It is quite close to the limb tonight. In the animation below, shown without the phase, the orientation of the Moon is not changed by the position in the sky or the latitude of the observer. Created using Stellarium.
A demonstration of libration by viewing the position of Mare Crisium. Also note that there is also a libration in the Moon’s latitude, causing a north-south nodding. Another good indicator of libration is the dark floored crater called Grimaldi on the left side of the Moon, which is not visible tonight in the image at the top. But it will appear in sunlight tomorrow night. Created using Virtual Moon Atlas.
12/14/2021 – Ephemeris – The Moon tonight will reveal more than you think
This is Ephemeris for Tuesday, December 14th. Today the Sun will be up for 8 hours and 49 minutes, setting at 5:02, and it will rise tomorrow at 8:13. The Moon, halfway from first quarter to full, will set at 4:45 tomorrow morning.
The gibbous Moon tonight is revealing a bit more of itself. In binoculars the dark oval spot visible on the Moon’s right side is the Sea of Crises or Mare Crisium a small dark lava plain. The Moon’s rotation is quite uniform, however its orbit isn’t circular, so the Moon’s face seems to rock a bit back and forth over the month. It’s an effect called libration. And one way to track that is to note how close the Sea of Crises is to the edge of the Moon. Right now that sea is as far from the Moon’s right edge or limb as it gets, and reveals two other seas in the edge: Mare Marginis, the Border Sea, and Mare Smythi, Smith’s Sea.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EST, UT – 5 hours). They may be different for your location.
Addendum
The Moon showing Maria Marginis and Smythi past Mare Crisium at 9 pm tonight, December 14, 2021. Created using Stellarium.
The Moon, seen as if flying over the border area of what we could see from the Earth. To the left is the Moon’s near side. To the right is the far side to the terminator or sunset line. Created using Virtual Moon Atlas.
Both the Stellarium and Virtual Moon Atlas apps are free. Links to them are elsewhere on this page.
11/12/2021 – Ephemeris – The Moon appears to wobble slowly over the month
This is Ephemeris for Friday, November 12th. Today the Sun will be up for 9 hours and 41 minutes, setting at 5:17, and it will rise tomorrow at 7:37. The Moon, 1 day past first quarter, will set at 1:27 tomorrow morning.
When I look at the Moon, I check the position of one particular feature. A small, nearly circular gray area near the right or western edge of the Moon’s disk. It’s called the Sea of Crises, or in Latin, Mare Crisium. The reason I check it is that over the month its distance from the edge, or what astronomers call the limb, changes. That sea is about as far as it can get from the limb now. The reason for this wobbling back and forth, called libration, is that the Moon rotates at a constant rate due to angular momentum, but its revolution around the earth is an ellipse, so it doesn’t move with a uniform speed in its orbit. It moves faster at its closest to the Earth, called perigee, and is slowest at apogee, its farthest from the Earth.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EST, UT – 5 hours). They may be different for your location.
Addendum.
The Moon tonight (November 12, 2021) showing Mare Crisium (Sea of Crises) about as far from the limb (Moon’s edge) as it can get. Created using Stellarium.
Simulation of the Moon’s phase and libration for October 2007 by Tomruen. Image is in the Public Domain.
04/15/2021 – Ephemeris – The Moon tonight – libration
This is Bob Moler with Ephemeris for Thursday, April 15th. Today the Sun will be up for 13 hours and 30 minutes, setting at 8:28, and it will rise tomorrow at 6:56. The Moon, 4 days past new, will set at 12:14 tomorrow morning.
The crescent Moon tonight is in the constellation of Taurus the bull. The bright star Aldebaran will be just to the left of the Moon. In binoculars the dark oval spot visible on the Moon is the Sea of Crises or Mare Crisium a small dark lava plain. The Moon’s rotation is quite uniform, however its orbit isn’t circular, so the Moon’s appearance seems to rock a bit back and forth over the month. It’s an effect called libration. And one way to track that is to note how close the Sea of Crises is to the edge of the Moon. A week from now that sea will appear its closest to the edge. Astronomers call the edge of the Moon the limb. The phase line between day and night on the Moon is called the terminator. Now as the Moon is waxing, it is the morning or sunrise terminator.
The astronomical event times given are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
11/08/2016 – Ephemeris – As the Moon wobbles
Ephemeris for Election Day, Tuesday, November 8th. The Sun will rise at 7:31. It’ll be up for 9 hours and 50 minutes, setting at 5:21. The Moon, 1 day past first quarter, will set at 1:14 tomorrow morning.
Tonight’s Moon bears close examination. The Moon has an elliptical orbit of the Earth, so its motion around the Earth moves fastest when nearer and slower when farther from the Earth. The Moon’s rotational speed, however is constant, Because of this the Moon appears to wobble back and forth. It also nods a bit. The effect is called libration. As of yesterday the Moon was rotated its greatest to the right for the month. It’s a small amount, only 8 degrees. On the 18th only 10 days from now the moon will nod 8 degrees to the left. A good way to tell the state of this libration for the waxing Moon is to check the position of the small gray sea near the right edge of the Moon, the Sea of Crises. Now it’s right at the edge of the Moon.
Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
Simulation of the Moon’s phase and libration for October 2007 by Tomruen. Image is in the Public Domain. Downloaded from the Wikipedia entry for the Moon.
You can run a continuous simulation like this using the free program Virtual Moon Atlas.
04/11/2016 – Ephemeris – The margins of the Moon
Ephemeris for Monday, April 11th. The Sun will rise at 7:04. It’ll be up for 13 hours and 18 minutes, setting at 8:23. The Moon, 2 days before first quarter, will set at 1:28 tomorrow morning.
Lets check out the Moon tonight. The Moon’s terminator, now the sunrise line traversing the Moon is beginning to uncover the dark Mare Tranquillitatis or Sea if Tranquility on the equator of the Moon. The small round prominent sea on the right side of the Moon is Mare Crisium, or Sea of Crises. At the limb beyond Crisium is Mare Marginis or sea at the Margin, and below it also at the edge is Mare Smythii, or Smith’s Sea, named after William Henry Smith a 19th century British astronomer. These last two seas are only visible when the Moon appears to rock to the left and reveals them. This is called libration caused by the fact the Moon rotates at a constant rate and it orbits the Earth in an ellipse, changing it’s speed.
Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
Map of the Moon for April 11, 2016. The arrow shows the point of maximum libration for that date. Created using Virtual Moon Atlas.
Photo of the receding Moon taken by the Crew of Apollo 8 as they started to head home showing the near side on the left and the far side on the right. I’ve annotated the seas which I’ve described above. Credit NASA.
02/06/2014 – Ephemeris – The moon tonight and libration
Ephemeris for Thursday, February 6th. The sun will rise at 7:55. It’ll be up for 10 hours and 3 minutes, setting at 5:58. The Moon, at first quarter today, will set at 2:11 tomorrow morning.
The Moon, being at first quarter today, has the terminator or sunrise line cut the Moon in half tonight. In binoculars and telescopes the best detail is seen near the terminator where the shadows are longest. The farther one wanders with a telescope the flatter the terrain appears. There’s another effect that can be seen. It’s called libration where the Moon appears to slowly wobble during the month. This is mostly caused by the Moon’s very steady rotation countered by the fact that the Moon’s orbit is elliptical and gets ahead or behind the rotation. At the upper right of the Moon is the small Sea of Crises. That has rotated forward a bit revealing yet another sea at the edge of the Moon, Mare Marginis, the Margin or Border Sea.
Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Addendum
The Moon highlighting the Ephemeris topics at 9 p.m. on February 6, 2014. Created using Virtual Moon Atlas.
The Moon highlighting the Ephemeris topics at 9 p.m. on February 6, 2014. Created using Virtual Moon Atlas.
10/11/11 – Ephemeris – The Hunters Moon and libration
Tuesday, October 11th. The sun will rise at 7:52. It’ll be up for 11 hours and 13 minutes, setting at 7:05. The moon, at full today, will rise at 6:34 this evening.
Tonight’s full moon is the hunter’s moon, and it has some of the characteristics of last month’s Harvest Moon. That is the moon rises less than the average 50 minutes later each night. Tomorrow the moon will rose 26 minutes later than tonight. The moon rotates in the same time that it orbits the earth. Only the moon’s rotation is constant, but its orbit around the earth is elliptical, and is not at a constant speed. That causes the moon to wobble a bit. Tonight the south end of the moon is tilted a bit toward is as is the east or left edge. This tilt is called libration. I usually track libration by checking out the position of the small gray sea at the moon’s upper right quadrant called the Sea of Crises. It’s distance from the moon’s limb or edge is a good indicator of this libration.
* Times are for the Traverse City/Interlochen area of Michigan. They may be different for your location.
Bob Moler's Ephemeris Blog 