We all know the basics of the Diurnal Cycle – day and night, sunrise and sunset. And we are all aware that during the day, the Sun is the most luminous object in the sky, to the point that it completely obscures the stars. And at night, the Moon (when it is visible) is the most luminous object, sometimes to the point that it can make gazing at the Milky Way and Deep-Sky Objects more difficult.
This dichotomy of night and day, darkness and light, are why the Moon and the Sun were often worshiped together by ancient cultures. But at times, the Moon is visible even in the daytime. We’ve all seen it, hanging low in the sky, a pale impression against a background of blue? But just what accounts for this? How is it that we can see the brightest object in the night sky when the Sun is still beaming overhead?
Simply put, there are two reasons for why the Moon can be seen during the daytime. First, there is the Moon’s apparent luminosity, which is due to its proximity to our planet and a combination of other factors. Second, there is the particular nature of the Moon’s orbit around Earth, otherwise known as the Lunar Cycle. Between these two factors, the Moon can become visible to the casual observer during the day.
As we are all no doubt aware, the Moon is the closest stellar object to Earth. Ranging from 356,400 – 370,400 km at Perigee (i.e. when it is closet to Earth) to 404,000 – 406,700 km at Apogee (farthest from Earth), it is about 104 to 644 times closer to Earth than the next nearest object. This would be the planet Venus, which ranges in distance from 38 million km at its closest to 261 million km at its farthest.
This close proximity to Earth is what makes the Moon brightest object in the sky, at least when the Sun’s not around. It also gets a boost due to what is known as the Opposition Effect, which refers to how an object can appear brighter when it is illuminated from directly behind the observer. It also appears brighter due to the fact that it is surrounded by the dark sky.
Last, but not least, there is the effect of the Lunar soil itself, which prevents a phenomena known as limb darkening from occuring. Basically, because the soil reflects more light back towards the Sun than in other directions, the center of the Moon appears just as luminous as its outer edges.
As stated already, the other reason for the Moon’s apparent brightness is due to the nature of its continuous orbit around Earth. Like most moons, ours is in synchronous rotation with the planet, which means one side is constantly facing towards the Earth. The Moon makes a complete orbit around Earth every 27.3 days (aka. a sidereal period), but takes about 29.5 days to appear in the same phase in the sky (its synodic, or orbital, period).
Over the course of the Moon completing an orbital period, it goes through eight phases – i.e. changes in appearance – that give an indication of where it is in its cycle. Essentially, the sun always illuminates only half of the moon, which we see from different angles as it rotates around the Earth. At the beginning of the cycle, we cannot see the Moon because none of its illuminated side is pointed towards Earth. This is known as a “New Moon”.
Over the course of the next 29.5 days, the Moon will go through the phases of Waxing Crescent, First Quarter (i.e. a “half-Moon”) and Waxing Gibbous before appearing as a Full Moon. It will then complete the cycle by going through the phases of Waning Gibbous, Third Quarter, and Waning Crescent before returning to a New Moon. In each phase, only a portion of the illuminated half is visible to Earth, ranging from 0% at a New Moon to 100% during a Full Moon
In addition, the Moon’s orbit around Earth also means it’s distance from the Sun changes over time. The reason the moon is at its brightest during a Full Moon is simply because it is during this phase that it is exactly opposite the Sun. However, as it gets closer to the New Moon phase of its cycle, its distance from the Sun decreases. This means that while it is less visible at night, during the daytime, it is more luminous.
And there you have it. The Moon can sometimes appear in the sky during the daytime because it is the closest object to Earth, and because its orbital cycle means that sometimes it is brighter in the day than at night. From the conjunction of these elements, people are sometimes able to catch sight of a beautiful, faint Moon hovering on the horizon.
We have written many interesting articles about the Moon here at Universe Today. Here are some Interesting Facts About The Moon, How Long It Takes To Get To The Moon?, and an article about Water On The Moon.
We’ve also recorded an entire episode of Astronomy Cast all about the Moon. Listen here, Episode 113: The Moon, Part 1.