Tonight – February 15, 2017 – the moon and planet Jupiter are still close in Earth’s sky. The bright star Spica in the constellation Virgo is also nearby. Watch for them late tonight, or early in the morning on February 16. You can’t miss the moon and Jupiter because they’ll be the two brightest objects in the sky in the late-night hours, and tomorrow before dawn. Another planet, Venus, is brighter than Jupiter, but it’s setting shortly after sunset now.
Did you see the moon and Jupiter last night or this morning? If you did, you might notice the moon has moved with respect to this planet and star. Night by night, as we stand on Earth’s surface watching the moon, we can see it shift its position in front of the fixed stars and wandering planets. This motion is due to the moon’s actual motion in orbit around Earth.
The moon takes about a month (one moonth) to orbit the Earth. Although the moon rises in the east and sets in the west each day (due to Earth’s spin), it’s also moving on the sky’s dome each day due to its own motion in orbit around Earth.
The moon’s orbital motion can be detected in the course of a single night, but you have to watch the moon closely, with respect to stars in its vicinity, over several hours.
The moon’s eastward, orbital motion is easiest to notice from one day (or night) to the next. It’s as though the moon is moving on the inside of a circle of 360 degrees. The moon’s orbit carries it around Earth’s sky once a month, because the moon takes about a month to orbit Earth.
So that the moon moves – with respect to the fixed stars – by about 12-13 degrees each day.
Bottom line: Did you see the moon and Jupiter last night or this morning? If you did, you’ll see the moon has shifted its position tonight with respect to this planet. The reason? The moon’s orbit!