Expert: Tom Whiting - 3/30/2010

Question
Mr. Whiting

You've answered question related to the earth viewed from the moon. But does this depend on the time of the day? I mean if we are in the morning or afternoon (on the moon facing the earth), how we see the earth? Does we see it illuminated? At what time we see it dark?

Answer
Hi Chloe,
A lunar observer on the moon viewing the Earth sees a stationary (but rotating) Earth in the lunar sky. If observer is on the lunar equator, the Earth is directly overhead; If observer
is at a lunar pole, then the Earth continuously sits right on the horizon. In effect, the Earth would be the "north star" of the moon... you could actually determine your position on the lunar
surface, the Earth-facing side, just by observing the stationary (but rotating) Earth and determine your position on the moon with respect to the Earth's position in your lunar sky.
(Just like our Polaris, from the Northern hemisphere at any one location, Polaris is always in
the same place in the sky, day or night- it never moves from it's position over one lifetime).

The Earth phase angle you see would be the opposite of what Earthling's view of the moon.
When Earth observers see a full moon, a lunar observer is looking back at a "new (read that dark) Earth" with the sun in the background, nearby, somewhere.  If perchance Earthlings are viewing a total eclipse of the moon, then the lunar observer is watching a total eclipse of the sun by the Earth passing directly in front of the solar disk. But most of the time, the sun
would pass either above or below the dark, "new" Earth. If Earth observers are viewing a first
quarter moon, then lunar observers are viewing a last quarter Earth. The big difference is...
our first quarter moon will rise and set once every 24 hours because we have a fast rotation,
whereas the last quarter Earth from the moon remains stationary in the sky from any one location.
So the Earth goes through it's phases, without moving in the Lunar sky.  Of course, if you, the
lunar observer move, say from a polar region to the lunar equator, then the Earth will appear
to climb higher in the lunar sky, but that's you moving on the moon, not the Earth moving.
(Just like Polaris gets lower in the northern sky if you head southward on the Earth's surface).

The time of "day" on the moon has nothing to do with the Earth's position in the lunar sky
That would all have to do with the sun's position in the lunar sky. ON the moon, they would have 2 weeks of daylight and 2 weeks of no sunlight at any one location (not counting extreme polar regions). But the Earth is stationary in the lunar sky, and the stars and the sun would
stream behind the stationary Earth as the month goes by, while the Earth changes it's "phase"
as the sun appears to move completely around the moon.
So yes, the Earth would always be illuminated to some extent, except at a "new Earth" at which time a lunar observer would see a dark earth with the sun very nearby it, in a high-noon, sunlit sky.
And during the lunar night on the Earth-facing side (naturally the sun is below your lunar horizon lighting up the backside of the moon), the full Earth would illuminate the dark lunar landscape with about the light of several of our full moons.  So there never is a total darkness on the Earth-facing side of the moon, but there really is total darkness for 2 weeks on the backside of the moon...because they, on the lunar backside, never can view the Earth, as the moon keeps the same side facing the Earth all the time.  Not counting minor perturbations and librations of the moon, of course... which would only be visible from an extreme lunar limb, or polar, observer.) It's not a perfect 1:1 rotation/revolution relationship, called synchronous
rotation... at times during the month the rotation gets slightly ahead of the revolution, and then behind it, so in reality, a keen eyed careful observer would see the Earth move (drift) very slightly from side to side a very small amount during the one lunar month.) And due to
lunar librations, the earth would drift very slightly up and down too, a very small amount.
Only a very careful lunar observer would notice these very slight movements... and it's not the
Earth moving, it's the whole moon doing it.
Hope this helps,
Clear Skies,
Tom Whiting
Erie PA USA