Earth & Sky

Moon well below Spica on evening of May 17

Tonight's Sky for Saturday, May 17 2008

Tonight’s moon is waxing toward full, but full moon is still a couple of days away. Tonight, the moon is still in a waxing gibbous phase when it appears over your southeastern horizon in early evening.

The moon was very near Spica last night, and it’s still near this star tonight – though, as always, the moon has now moved eastward in its orbit around Earth and so appears farther to the east on our sky’s dome. As the moon moves in orbit around Earth, it travels some 13 degrees eastward in front of the background stars in 24 hours. (For reference, the moon’s angular diameter equals 1/2 degee.)

Meanwhile, the stars are considered fixed relative to each other. That’s why we see the same constellations as the early stargazers. The patterns of the constellations don’t change, and yet the stars do shift as the seasons pass, and Earth’s night sky points out in an ever-changing panorama of the galaxy Our planet’s motion aro0undt he sun ios what causes the stars to rise and set 4 minutes earlier each day.

The moon’s motion around Earth is what causes the moon to rise and set about 50 minutes later each day.

Written by Deborah Byrd, Bruce McClure , 17 May 08