Earth rise on the moon ... common

Practically all of us know that the Moon is always turned to Earth by the same side. From school physics courses, we also know that the reason for this is the Earth tides, which have forever hidden from us the opposite, "dark" side of the Moon. The principle of tidal capture postulates that the planet - the hostess is almost always at one point in the sky of his satellite. However, I said this too unambiguously, because in reality this is possible only under ideal conditions. The world, to our happiness, is far from ideal, which completely allows us to observe full sunrises and sunsets on the moon ...


Astronomers have long noticed that the moon peculiarly "sways" during the lunar month, exposing to us up to 10% of the area of ​​the "dark" side. As a result, before the flight of the station "Luna 3", astronomers had maps of 60% of the lunar surface.
This phenomenon was called libration. At the moment, there are 4 types of librations, but we will focus on the two main ones - librations in latitude and longitude.

1.Librations in latitude are caused by the inclination of the axis of the daily rotation of the moon to the plane of its orbit (amplitude 6 ° 50 min), as a result of which the moon “substitutes” either the north pole or the south pole.
2. Longitude librations are not caused by the zero eccentricity of the lunar orbit.
The eccentricity of the orbit in a simplified version displays the degree of deviation of the satellite or planet orbit from the ideal circle. 0 means a perfectly round orbit. More than 0, but less than 1, in one degree or another elongated orbit (elliptical), with e = 1 parabolic, and with e> 1 - hyperbolic. As you have noticed, the orbit is gradually drawn out as the eccentricity increases from 0 to 1, being torn to e = 1 (reaching the second space in this orbit).
')
Librations of the Moon, view from the Earth.


The eccentricity of the moon is on average equal to 0.05, which is quite enough for the appearance of small deviations between the speed of rotation of the moon around the earth and its own rotation of the moon around its axis. This provokes longitudinal libration with an amplitude of 7 ° and 54 min.

It is obvious that both types of libration cause the movement of the Earth in the sky of the Moon - where the blue planet describes a huge ellipse with the largest diameter of 18 ° during the month. Given that the angular size of the Earth from the Moon is “only” about 2 ° (four times larger than the size of the Moon seen from the Earth), this will allow future lunar colonists to observe even slow, but spectacular sunrises and sunsets of the home planet in certain areas of the Moon.

Earth's rise in the "zones of libration", the moon pole, mid-latitudes and the equator (Stellarium program).


However, the least patient colonists can easily observe this “in fast rewind” from the moon's orbit (Kaguya / JAXA probe).


And a small bonus. Although on Yapet, the satellite of Saturn, most likely there is no stargate where the hero of Arthur Clarke’s Space Odyssey 2001 managed to please, but still due to the irregularities of the orbit of this satellite, one can observe quite epic Sunrises of The Lord of the Rings.