FAS Astronomers Blog, Volume 32, Number 1.
One of our favorite observation targets is the Moon. Most of the time, we observe it at night. Interestingly, the best time to see features on the Moon isn’t during a full moon – it is much too bright, but during a first or third quarter when you can see objects along the terminator between the dark and light side.
The Moon revolves around the Earth every 27.3 days, but goes through its series of phases every 29 1/2 days. The additional time is due to the Earth’s (and Moon’s) orbit around the Sun.
When the Moon is directly in line with the Sun, the far side is illuminated and we have a new moon. At this time, the Moon is in the sky all day long, but it isn’t visible. When the Moon is on the opposite side of the Earth from the Sun, the near side is illuminated and we have a full moon. In this case the Moon is visible throughout the night, but not during the day.
When the Moon’s phase is either a waxing or waning gibbous, a large portion of the near side is illuminated, and it is visible during the day. This is when we see a “daytime Moon.”
- A waxing gibbous occurs just before a full moon and is visible in the afternoon to the west.
- A waning gibbous occurs just after a full moon and is visible in the morning to the east.
Selected Sources and Further Reading
- Katrin Raynor. “Why do we sometimes see the Moon during the day?” BBC Sky At Night Magazine. November 30, 2023.
- Martin McGuigan. “Why can we sometimes see the moon in the daytime?” Live Science. October 30, 2022.
- Tricia Talbert. “Why Can You See the Moon During the Day? We Asked a NASA Scientist: Episode 19.” NASA. January 5, 2022.
- Deborah Byrd. “When can you see a daytime moon?” EarthSky. January 29, 2013.
- “Daily Moon Guide.” NASA Science, Earth’s Moon.
- “Observing the Moon and Planets.” FAS Astronomers Blog. October 2021.
- “The Night Sky.” FAS Website.
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