Around the Sky in 28 Days – Day 3
September 11, 2010
Last night the Moon was to the right and below Venus and Mars. Tonight it has moved to the left of the two. Still the Moon is close enough to be used as a guide to easily find Venus, Mars and the bright star Spica.
This evening the Moon is about 10% illuminated meaning that only ~1/1oth of the Moon’s disk is in sunlight as seen from Earth. Yet, if you look closely the entire disk is visible. Why is this? Have you ever noticed how bright the landscape is when the Moon is Full. Well, right now on the Moon, the Earth is nearly Full and is illuminating the night side of the Moon. The faint glow we see is called Earthshine.
The brightness of the earthshine is not constant and changes depending on the amount of snow and cloud cover on Earth. Researchers have even been able to measure changes in Earth’s cloud cover by measuring the brightness of Earthshine on the Moon. Amazing…
Coming tomorrow – Day 4 – Its time to take a closer look at the Moon.