Posts Tagged ‘Mare Smythii’

12/14/2021 – Ephemeris – The Moon tonight will reveal more than you think

December 14, 2021 Comments off

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.


Moon showing libration

The Moon showing Maria Marginis and Smythi past Mare Crisium at 9 pm tonight, December 14, 2021. Created using Stellarium.

Moon as seen if over Mare Marginis

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.

04/11/2016 – Ephemeris – The margins of the Moon

April 11, 2016 Comments off

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.


Moon April11, 2016

Map of the Moon for April 11, 2016. The arrow shows the point of maximum libration for that date. Created using Virtual Moon Atlas.

Apollo 8 image annotated

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.