Why do we see the Moon during the day only on certain days and not every day?
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3 Answers
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The Moon is in orbit around the Earth, and takes 29.5 days to complete its circle. This means that it passes the Sun in the sky once every 29.5 days (called New Moon) and then moves slowly away from the Sun until it's exactly opposite the Sun at Full Moon, two weeks later. At any point in these two weeks, you can see the Sun and the Moon in the afternoon sky at the same time. At Full Moon, the Moon rises in the east at exactly the same time as the Sun sets in the west, and that's the only night in the whole month when the Moon is in the night sky all night long. After Full Moon, the Moon continues in its orbit, moving slowly towards the Sun on the opposite side, and again Sun and Moon can be seen at the same time in the morning sky. After another two weeks, the Moon approaches the Sun closely, and is lost in the Sun's glare for a few days. So, basically, having both the Sun and the Moon in the sky simultaneously is the normal thing, almost every day of the month, while having the Moon only in the night sky happens only one night a month.
The fact that so few people have observed the Moon and Sun in the sky at the same time merely confirms that most people rarely if ever look at the sky.
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$\begingroup$ Correct answer. We don't notice the moon during the day as often because there is less contrast between it and the daytime sky, often cloud cover, etc. At night the moon actually provides significant illumination, so we are more likely to notice it. The USNO has a web page that calculates the moon's rise and set time. $\endgroup$– ghoppeCommented Jan 24, 2012 at 22:15
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2$\begingroup$ To clarify, 29.5 days is not Moon's orbital period, but rather the period of its illumination cycle (synodic period). Moon's orbital period, from fixed star to that same fixed star, is 27.3 days. $\endgroup$– user11266Commented Jan 25, 2013 at 2:03
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$\begingroup$ Put in another way (slightly simplified of course): The new moon will rise and set approximately at the same time as the sun, so it will be in the day sky all day (but very hard to see!). The waxing moon will rise during the day time, and will be visible in the afternoon/evening; it sets during the night. The full moon rises and sets approximately opposite of the sun, so will be visible all night, but not in the day. The waning moon rises during night, is visible in the morning, and sets during the day. See table at lunar phase. $\endgroup$ Commented Mar 27, 2015 at 10:05
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Contrary to common believe, the moon and sun are completely independent as to which is in the sky at any moment. The Moon moves through the sky every 24 hours, plus it moves approximately 1/29th extra per day (The moon rotates around the Earth every 29 days, thus every day it will move by 1/29th of it's total path). Thus, the moon will appear in the same spot every 24 hours and 50 minutes or so.
So, what does that mean? Simply put, there is a 50% chance that the moon will be somewhere in the sky at any moment in time, and a 50% chance that the sun will be in the sky. Thus, half of the time that the moon is in the sky, it will be day.
Why then, one might ask, is the moon most commonly thought of as visible only at night? There are 2 primary reasons.
- At night, spotting the moon is very easy, as it is the brightest object in the sky by far.
- The moon is more likely to be full at night, and new when it is seen during the day. A full moon happens when the moon is on the opposite side of the sky from the sun, and a new moon when it is on the same side of the sky as the Sun. So, the moon is brighter when it is easier to see it at night.
I hope all of this helped a bit to explain this. Let me know if I can help you understand further.
answered Jan 9, 2012 at 3:23
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$\begingroup$ I am still unsure why I only see it on certain days are there other issues they would change it being visible or not? $\endgroup$– YUASKCommented Jan 9, 2012 at 4:07
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$\begingroup$ In order to see it, it has to be above the Horizon, and bright enough to be where you can see it, and you have to pick it out from the Sun's glare. It is above the horizon half the time during the day, but the last 2 points are tricky during the day. $\endgroup$ Commented Jan 9, 2012 at 4:16
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$\begingroup$ is there a video that illustrates how this happens I seem to still not grasp it $\endgroup$– YUASKCommented Jan 9, 2012 at 23:50
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$\begingroup$ While largely correct, this answer does not account for the seasons. The 50% chance the sun is in the sky is actually closer to 30% where I live right now :) $\endgroup$– ghoppeCommented Jan 24, 2012 at 22:19
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1$\begingroup$ Check out this moon phase simulator. In particular, the box labelled horizon diagram, showing the path of the Sun and the Moon, may be helpful for visualizing. $\endgroup$– jdmcbrCommented Feb 2, 2012 at 0:15
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I'll jump on this wagon.
There is a natural occurrence called "Earthshine." This is because the earth will reflect some of the suns light at about 4 times the brightness of the moon at night. This allows the moon to have light hit it and be visible during the day.
Now as for the rotation of them moon, it passes through the sky and will go completely around it's orbit in about 29 days. This means that on some days the moon will be shown during the day (ie: before the waxing First Quarter and after the Waning Second Quarter... in between these phases is the New Moon).
Hope that makes sense!
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1$\begingroup$ Earthshine is not what makes the moon visible during the day - it is the sun's rays reflecting off the moon that make it visible. Earthshine is what lets you see (dimly) the dark areas of the moon at night $\endgroup$ Commented Feb 2, 2012 at 11:22
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1$\begingroup$ @RoryAlsop It's both. Finding the Moon in daylight without the sun illuminated portion would be very difficult; but the portion of the disk only illuminated by Earthshine is also visible. $\endgroup$ Commented Feb 9, 2012 at 15:43