Tonight – on September 23, 2016 – you won’t see the moon and the Gemini stars, Castor and Pollux, at nightfall or this evening. From most of the world, you’ll have to wait until midnight or later to see the waning crescent moon and the Gemini stars climbing over the eastern horizon. Your best bet is to get up before dawn to view the moon and the Gemini stars, when the moon, Castor and Pollux are much higher up in the sky.
After the moon, Castor and Pollux rise tonight, they’ll go westward and upward throughout the wee morning hours for the same reason that the sun goes westward during the day. The Earth spins eastward beneath the heavens once a day, causing the sun, moon, planets and stars to rise in the east and to set in the west each day. Any celestial object – such as the sun, moon, planet or star – transits – reaches its highest point in the sky – midway between rising and setting
If you’d like to know when any major solar system body rises/sets/transits in your sky, check out this U.S. Naval observatory page.
The moon actually moves eastward relative to the stars of the zodiac (such as zodiacal stars Castor and Pollux), even as it moves westward across Earth’s sky each day. Therefore, the moon will be farther east in front of the constellation Gemini on September 25 than at the same time on September 24.
On the average, the moon moves 13o eastward in front of the constellations of the zodiac each day. (For reference, the diameter of the moon spans about one-half degree.) So, on the average, the moon rises and sets about 50 minutes later daily. The moon’s change of position relative to the backdrop stars is due to the moon’s orbital motion around Earth.
The stars rise and set about four minutes earlier each day. That’s because the Earth in its orbit goes about one degree around the sun, slightly changing our perspective of the starry heavens from day to day.
Each day, the moon moves an average of 12o eastward of the sun – yet 13o eastward in front of the backdrop stars of the zodiac. That’s because each day, as seen from Earth, the sun appears to travel one degree eastward in front of the backdrop stars.
Enjoy the moon and Gemini stars, Castor and Pollux, as the moon makes its monthly journey through the zodiacal constellation Gemini the Twins over the next few days.