Why can I see the Moon during the day? Part 2

Question Answered by Dr. Brian Tonks, Astronomy Professor, BYU-Idaho

Check out Part 1 of this question at http://www.idahoaskascientist.com.

Part 2: Now let’s add the Moon to the figure at the right . We have to be careful when we look at figures like these because they are never drawn to the correct scale. If the Earth were the size of a 12 inch globe, the Moon would only be 3 inches in diameter (about ¼ Earth’s diameter). However, that 3 inch globe would be 30 feet away from the 12 inch Earth! Try to draw that on a normal sheet of paper! These figures don’t capture that scale. In this figure, notice that the Moon is directly across from the Sun. From our position on Earth, notice that we can’t see the Moon at all during the daytime—we’d have to be able to look through the Earth. Now maybe Superman can do that, but we mere mortals cannot! Also notice that the entire lit up face of the Moon is visible from Earth. We call this a “Full” moon. We can only see a full Moon at night.

The figure below shows the Moon when it’s about halfway between the Sun and Full Moon. I’ve also shown our observer at four different times of the day. When the observer is at position a, the moon is just rising. The Moon would be visible at all times when the observer is between a and c. That means you would be able to see the Moon during the afternoon and into the early evening hours (until about midnight). If the angle between the Sun, observer, and the Moon is 90o (as shown for the observer at point b), we see half of the Moon’s bright face. The Moon would appear half full. Astronomers call this phase “First Quarter”. We would not be able to see the Moon once we rotate beyond point c. At point d, we’d have to look through the ENTIRE solid Earth to see the Moon (I’m not sure that even Superman can do that). The main point is that First Quarter moon is visible during the afternoon hours. You see the Moon during the day.

Because the Moon orbits the Earth, at some point during the month, the Moon will be at every one of the positions shown in the figure at right (and the points in between them!). At point 1, the Moon lies directly between the Sun and Earth. Of course, the Moon’s dark face points towards Earth. We call this phase “New Moon”. You would only be able to see this phase during the day. The only time we can see a New Moon is when it crosses the Sun’s face. This happened in 2017 when we experienced the total solar eclipse in our region.

In positions 2 and 8 we see at least a sliver of the Moon’s lit-up surface. These moons have crescent shapes. Notice that we see them mostly during the daytime. We see crescent moons during the day, but you have to paying attention them because they are quite close to the Sun. It’s more likely that we’ll see the Moon during the day in positions 4 and 6. In these positions the Moon is larger than half but less than full. This shape is called a “gibbous”. We see the Moon at position 4 rising in the late afternoon and it remains visible during the night hours when we’re still awake. The gibbous moon in position 6 rises in the late evening (possibly after your bedtime), and is visible the remainder of the night and into the early morning. We’ll often see this Moon in the western sky during the morning. At position 8, the Moon is far enough away from the Sun so we can see it easily. It rises early in the morning (3 am or thereabouts). It will be visible during the morning hours and into the early afternoon.

Thus, we can see all phases of the Moon, except New Moon and Full Moon during at least some of the daylight hours.

International Observe the Moon Night – October 5th
International Observe the Moon Night is a worldwide celebration of lunar science and exploration held annually since 2010. One day each year, everyone on Earth is invited to observe and learn about the Moon together, and to celebrate the cultural and personal connections we all have with our nearest neighbor.
Download NASA’s Moon Observation Log at https://moon.nasa.gov/resources/12/moon-observation-journal/ to participate.

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