Courtesy of EarthSky
A Clear Voice for Science
Tonight the moon is in a waxing gibbous phase, meaning it appears more than half lighted but less than full. Tonight, the moon is to the west of the bright star Spica and to the east of the planet Saturn. Yesterday evening, the moon was closer to Saturn. Tomorrow evening, the moon will be closer to Spica.
Spica is the brightest star in the constellation Virgo the Maiden. We have also indicated the whereabouts of the constellation Corvus the Crow on today’s chart. While Virgo is large and rambling, and not at all easy to pick out, Corvus is very easy to identify as a small squarish figure on a dark, moonless night. You will always know you are seeing Spica when you see little Corvus nearby.
Now think about why this motion of the moon is predictable. It is because the moon moves in orbit around Earth! Month after month, year after year, the moon moves around and around our world – and so moves around and around on the dome of our sky.
This is also the answer to the following question: “Why do the times of the moon’s rising and setting vary during the month?” Moonrise and moonset are caused by Earth’s spin on its axis. Moonrise happens when the part of Earth you are standing on spins you around to the sight of the moon in space. Because the moon is moving toward the east in orbit around Earth, our planet has to spin a little longer each day to bring you around to where the moon is. Thus the moon rises and sets an average of 50 minutes later each day.
Written by Deborah Byrd