Earthshine
Published on Dec 5, 2005 at 7:55 pm.
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Filed under Earth, moon, skywatching.
Have you ever gone out and looked at the little crescent moon and seen the rest of the moon dimly lit? Science@NASA has a nice article on the earthshine that you can read. In fact, this is a very good article, and there isn’t much more that I can say.
Basically, earthshine is sunlight reflected from Earth onto the Moon. Leonardo Da Vinci produced a very good explanation for the phenomenon. Changes in weather patterns on Earth can change the amount of light reflected. The article that I reference talks about that. What it doesn’t really talk about is when is the best time to see earthshine.
Basically, you want the moon to be a small crescent. That means that it nearly lined up with the Sun, and so there is a nearly full Earth shining onto it. Now, you don’t want the moon to be too small of a crescent, because that would be too close to new moon. The moon would then set during twilight, and you’d miss out on the full impact of the earthshine. If it is too big of a crescent, or near a quarter phase, then the Earth would not look as full as seen from the Moon, so the earthshine would be not as bright. Coupled with the Moon itself being brighter, this makes the earthshine either tough to see, or simply not very impressive. The best time is usually a few days after new moon. Right about now, in fact. Now, the moon’s orbit around the Earth is not over the equator, so sometimes it is over the southern hemisphere and sometimes over the northern hemisphere. Right now, it is the former. This means that it is low in the sky for observers in the northern hemisphere, but higher in the sky for observers in the southern hemisphere. As the moon moves along its orbit, it will be too low to see earthshine as a small crescent for us in the northern hemisphere, so while the best time to observe the earthshine will come a day or so later than it will in the southern hemisphere. That makes it tonight as the optimal time to go look. The absolute best times to see earthshine would be when the moon is a small crescent but still far from the horizon. This happens about three days or so after new moon in March or April. You can see earthshine in the mornings, too, but the best time there will be a few days before new moon in September or October. (Reverse those months for the southern hemisphere). That doesn’t mean, though, that it won’t be cool to go look at tonight if you have clear skies.
-Astroprof
