Many people only notice our Moon at night, when there is considerably more contrast between the Moon and the night sky. Being the second brightest object in the sky (after the Sun, of course) and with Venus visible during the day to trained eyes, it’s no real surprise that the Moon is visible during the day.
Why then, do so many people act surprised when they notice the Moon during the day? What makes it possible for the Moon to be visible during the day?
Understanding how and when you can spot the Moon is a matter of knowing the different lunar phases, specifically the relationship between the Sun, Earth and the Moon during each phase. The image below shows the simple geometry responsible for each of the Moon’s distinct phases.
In the diagram it’s pretty easy to see that when Earth is between the Sun and the Moon, we see a full moon. When the Moon is between Earth and the Sun, we see a new Moon. The other phases are simply transitions from new to full and from full back to new.
Based on the orbital geometry of the Moon, there will certainly be times where the Sun will partially illuminate the Moon, during the day and at night. What makes the lunar cycle even more interesting is that the moon rises about an hour later each day, and yet invariably, a full moon rises near dusk and sets near sunrise. The reverse is true in that a new moon rises near sunrise and sets near dusk.
In the meantime enjoy the transition from waning gibbous to waning crescent over the next week and get your telescopes out during the weekend of the 25th. The Moon will almost be at its new phase. (UniverseToday)
If you’d like to learn more about moon phases and when the moon will be visible in your area, the US Naval observatory has a great calculator at: http://aa.usno.navy.mil/data/docs/RS_OneYear.php
The moon is orbiting the Earth once every 28ish days. You can’t see it during the day if it’s right between us and the sun because the back of it, which isn’t lit up by the sun, is pointed towards us. If the moon is around 45 degrees off the sun or even 90 degrees off the sun then half of it will be lit up really quite brightly. Then the surface of the moon is about as bright as the surface of the Earth is.
There’s no reason why you shouldn’t be able to see it! As long as the moon’s at 45 degrees the sunward side of the Earth and it’s lit reasonably you’ll be able to see it.
And if you can see the moon in your sky (whether it be day or night), rest assured that the people on the opposite side of the earth at that moment CANNOT see it in their sky (whether it be night or day).
The best way to understand what’s going on is to make some observations yourself. Try spotting the moon every day for about a month, and it’ll become clear. Some things you’ll notice:
On some days, the moon is rather close to the sun, and it forms a very thin crescent. Since it’s so close to the sun on those days, it rises and sets close to the same times when the sun rises and sets. This means it’s in the sky pretty much all day, and NOT in the sky pretty much all night. Since it’s close to the sun and it’s just a thin crescent, it may be hard to spot during the day. You may be able to catch it in the west just after sunset, or (if you’re an early riser) in the east just before sunrise.
On other days, the moon is rather far from the sun, and may be on the complete opposite side of the sky. On those days, the moon appears full or nearly full on account of its position relative to the earth and to its light source (the sun). Since it’s on the opposite side of the sky at those times, the moon rises in the east at more or less the same time that the sun is setting in the west. It will stay up more or less all night, and finally will set in the west at more or less the same time that the sun rises in the east.
You’ll also notice that the sun makes its way eastward over the course of a month. Try going out every night at the same time–say 7:00 PM, and notice whether you can see the moon, and where it is in the sky. If you catch it near the beginning of the lunar cycle, you’ll see that it’s in the west as a crescent (crescent always means it’s fairly close to the sun). The next night at 7:00 you’ll see that it’s farther east; the next night still farther east; and so on. Eventually it’s moved so far east that it will be near the eastern horizon at 7:00; and after that it will be below the horizon at 7:00. From that point you’ll have to wait another couple of weeks before you can see it in the west (at 7:00) again.
The Moon is in synchronous rotation: it rotates about its axis in about the same time it takes to orbit the Earth. This results in it nearly always keeping the same face turned towards the Earth. The Moon used to rotate at a faster rate, but early in its history, its rotation slowed and became tidally locked in this orientation as a result of frictional effects associated with tidal deformations caused by the Earth. The side of the Moon that faces Earth is called the near side, and the opposite side the far side. The far side is often called the “dark side,” but in fact, it is illuminated as often as the near side: once per lunar day, during the new Moon phase we observe on Earth when the near side is dark.
The Moon has an exceptionally low albedo, giving it a similar reflectance to coal. Despite this, it is the second brightest object in the sky after the Sun. This is partly due to the brightness enhancement of the opposition effect; at quarter phase, the Moon is only one-tenth as bright, rather than half as bright, as at full Moon. Additionally, colour constancy in the visual system recalibrates the relations between the colours of an object and its surroundings, and since the surrounding sky is comparatively dark, the sunlit Moon is perceived as a bright object. The edges of the full Moon seem as bright as the centre, with no limb darkening, due to the reflective properties of lunar soil, which reflects more light back towards the Sun than in other directions. The Moon does appear larger when close to the horizon, but this is a purely psychological effect, known as the Moon illusion, first described in the 7th century BC. The full Moon subtends an arc of about 0.52° (on average) in the sky, roughly the same apparent size as the Sun.
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