Do you remember a time when you looked up into the sky maybe in the middle of the day when the sun was shining brightly, and you saw the moon hovering over the horizon. The moon just sitting there, like a white ghostly image against the deep blue of the sky. That is something that many people experience and find fascinating. People who have noticed the moon during the day ask themselves questions such as “When is it going to go away?” or “Why do I only think of the moon at night?”
The answer is simply that the moon is in orbit around earth and thus it is visible in the daytime sky almost as frequently as it is at night. Understanding why this happens requires understanding what determines when we can see the moon during the day, including its orbit, its phases, and our location on earth.
Orbit Determines Visibility
We can easily understand why we can see the moon during the day. Simply stated, since the moon is always orbiting around the earth, it will be visible from somewhere on earth all of the time. The fact that it takes approximately 27.3 days to make one complete trip around earth explains why we may see it at various times of the day. If you stood still and observed the moon for an entire lunar cycle, you would see the moon rise and set at varying times each day. It is not a steady source of illumination that only comes on once the sun disappears below the horizon. The moon’s visibility is based upon its position relative to the sun and the earth.
Phases Determine Visibility
Additionally, there is another important factor that influences how well we can see the moon during the day. As previously explained, phases of the moon depend on how much of its sunlit surface we can see from earth.
New Moon
Since we cannot see any portion of the moon’s illuminated surface when it is between us and the sun (new moon), we cannot observe it during this period regardless of whether it is in conjunction with either the sun or stars. Thus, we should never expect to see a fully blackened new moon in daylight.
Waxing Crescent and Waning Crescent
When the moon begins to move away from its position in conjunction with the sun, a growing sliver of its sunlit face becomes apparent. This is known as a crescent moon. Since both the increasing crescent (the side that faces toward the sun increases) and decreasing crescent (the side toward the sun decreases) phases can usually be viewed during daylight, this accounts for additional periods when we can see the moon.
Gibbous Phases
Just as with crescents, we can also see gibbous moons (those that exhibit more than 50 percent of their illuminated surfaces) during daylight. You may recall viewing a large crescent or nearly full moon suspended in the sky later in the afternoon. Those instances are examples of daylight observations.
Full Moon and First/Third Quarters
A full moon occurs when the moon passes behind the sun in terms of longitude. Although it is possible to observe a full moon most often during nighttime hours, we can actually see it at other times as well. A full moon typically rises at sunset and sets at sunrise. Therefore, on a particular day when a full moon occurs, it is easy to locate in daylight hours by observing it setting in the west prior to sunrise. Alternatively, if you watch on a day immediately preceding the full moon, you can easily identify it rising in eastern skies late in the afternoon.
Daytime Observations Around First Quarter and Third Quarter
In addition to observing a full moon during daylight hours, some of our best opportunities to view the moon occur when it is in first quarter and third quarters. When the moon reaches first quarter, it rises about midday and reaches its highest altitude shortly after sunset. You will clearly see exactly half of its illuminated surface. On those occasions when it reaches third quarter (also referred to as last quarter), it rises about midnight and sets about midday, providing a good opportunity for morning viewing. Regardless of whether it reaches first or third quarter, during both of these phases, the angle formed between the earth, sun, and moon will be close to 90 degrees. As a result, they will be among the brightest objects we can view during daylight hours.
Visibility Due to Brightness and Sky Conditions
One final consideration affecting our ability to see the moon during daylight hours concerns its brightness and contrast with respect to the surrounding sky. While no light is produced by the moon itself, it reflects some of the sun’s light back onto earth. The amount of light reflected back by the moon varies depending on conditions present in space and on earth. On average, however, the moon reflects about 12 percent (average albedo = 0.12) of all sunlight hitting it. Fresh snow, on average, has an albedo ranging from 0.80 to 0.90. Despite reflecting less than 1/10th as much light as freshly fallen snow, its reflected light remains sufficient to be noticeable above a bright blue daytime sky. Our daytime sky is colored blue because of Rayleigh scattering. Shorter wavelengths of light (i.e., blue) are scattered more intensely by particles in our atmosphere than longer wavelengths of light (i.e., red). Consequently, short wavelength light tends to dominate our perception of blue-colored skylight. However, since reflected light from the sun (in this case from the moon) possesses greater intensity than that from scattered blue light, we are able to detect its presence.
Earth’s Rotation Affects Our Viewpoint
Finally, our viewpoint on earth affects our ability to view objects orbiting around us. Consider two objects: One that stands still on a merry-go-round while another object orbits around that merry-go-round. An observer on that merry-go-round would view that orbiting object from changing angles throughout an orbit around him/her.
The Earth rotates daily while the moon orbits. For about half of each lunation (monthly cycle), it exists above our horizon in daylight. This translates into approximately fourteen days per month when we ignore cloud cover or extremely thin crescent phases, we are able to view the daytime moon without difficulty.
Best Times for Viewing Daytime Moon
The best times for viewing daytime views of the moon are typically early morning or late afternoon. Early morning views allow observation of a daytime moon closest to first quarter positions and late afternoon views allow observation of a daytime moon closest to third quarter positions. Both of these situations provide favorable circumstances for viewing because at least one edge of a daytime moon is sufficiently distant from an edge of a direct ray from the sun to produce prominent contrast.
Common Misconceptions About Seeing Daytime Moon
Many people incorrectly assume that seeing a daytime moon is either rare or indicates some unusual astrophysical occurrence. This misconception stems from our tendency to pay more attention to a bright daytime sky compared to a dark nighttime sky. Additionally, daytime skies offer many competing visual attractions (e.g., intense sunlight) versus nighttime skies with few competing visual distractions.
Sources
- In Depth | Earth’s Moon — NASA Solar System Exploration
- Why can we see the Moon in the daytime? — Scientific American
- Moon Phases — Time and Date AS
- Daytime vs. Nighttime Moon — NASA Goddard Space Flight Center
- Rayleigh scattering — Wikipedia
