Why do we then not see the whole moon during the day?

Question

I understand that at night it's the earth blocking the reflection of the sun on the moon that causes partial moons,but what happens when we see partial moons during the day,seems like we should see a full moon?

Answer 1

Day or night, you are seeing the portion of the moon that is facing the sun. The only time Earth blocks illumination of the moon is during a lunar eclipse, which happens only two or three times a year.

Answer 2

Starting with the full moon - A day or two before the "full moon" you will see the
moon rise just before sunset. At full moon it will rise as the sun sets, and of course
after the sun sets as the time goes on. Each night after, notice how the light on the
moon shrinks from full to last quarter and then to nothing (a 2 week time period).
Also during this time you will notice the moon "traveling" closer to the sun.
Two weeks from full moon (day may vary) look for the sliver of the moon just before sunset.
As the moon moves into NEW moon and you cannot see at all, watch for it to become noticable
as a sliver in the other direction as the sun sets. Follow the light on the moon as
it expands to first quarter and then to full. So you can actually see the moon and the sun
at the same time virtually most of the month.

Just as a note, the earth shadow only affects the moon during an eclipse. The rest of the time the moon is outside the shadow.

Answer 3

You are seeing the day and night side of the moon. The moon only reflects light from the sun, and does not give off it's own light. The dark side is the side facing away from the sun. Next time during the day look up at the moon, and you will notice the side that is lit up is the side facing the sun.

Answer 4

In order to see a "full Moon" the moon has to be on the side of the earth opposite the sun, hence, it is only possible to see a full moon at night.

Answer 5

To help better understand, try holding a small ball (the moon) and a larger ball (the earth) in a room illuminated by a single light bulb.

Picture yourself standing on the lighted side of the big ball, then move the smaller ball around the bigger ball. You will notice that, depending on where you are standing (i.e. facing the light bulb or facing somewhere else), the small ball has various illuminations with respect to the large ball.

Answer 6

1. The Moon

The Moon is the only natural satellite of the Earth and orbits around the Earth. We can only see the Moon because it reflects light from the Sun, just like all other moons and planets are only visible because they reflect light from the Sun. The Moon is below the horizon (invisible) for on average half of the time, is visible during on average about half of the daytime, and is visible during on average about half of the nighttime as well.

Read the Sky and Horizon-page to find out about the phases of the Moon.

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The distance between the centers of the Earth and the Moon is about 384,400 km on average, but varies between about 356,400 and about 406,700 km.

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The Moon rises at a different hour each day, and also sets at a different hour each day, because the Moon moves quite a distance in the sky relative to the stars each day. See question 63.

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A method for calculating the approximate position of the Moon is described on the Calculation Page on Positions in the Sky.

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2. The Moon Has No Natural Moons

If the Moon has any natural moons of its own, then they must be really small. No such moons have ever been discovered. A moon with a diameter of 5 km (3 mi) in orbit around our Moon and made of the same material as the Moon would be as bright in the sky as the brightest stars, so such a moon would have been discovered long ago.

If such a moon had a diameter of 500 m (1500 ft), then it would be as bright as the faintest stars that one can see without binoculars or a telescope from a dark place, so such a moon would probably have been discovered by now as well.

Any moon orbiting around our Moon could not be further away from the Moon than about 35,000 km (22,000 mi), which is about 10 times the diameter of the Moon, because orbits beyond that distance would be disturbed so much by the gravity of the Earth that moons in such orbits would probably have escaped into space or fallen to the Moon or the Earth. (The "tidal boundary" between the Earth and the Moon lies about 35,000 km from the Moon.)

The Moon has had artificial moons: satellites such as Clementine and the Lunar Prospector that were sent from Earth into an orbit around the Moon to study the Moon. It is possible that some of those artificial satellites are still circling around the Moon, though they are usually deliberately crashed into the Moon once their mission is complete.

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3. The Moon Falls Towards But Not Onto The Earth

The Moon falls around the Earth all the time. If the Earth did not pull at the Moon, then the Moon would disappear into space, roughly along a straight line. The attraction between the Earth and the Moon pulls the Moon away from that straight line towards the Earth (and likewise pulls the Earth from a straight line towards the Moon), but not enough to make the Moon fall down unto the Earth.

In other words: The Moon has a large sideways speed (not directed towards the Earth, but rather perpendicular to that direction), so when the Moon has fallen a bit towards the Earth, then in the same time it has also moved sideways a bit, so that it is at about the same distance from the Earth as before.

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4. Daytime and Nighttime Moons

The Moon that we sometimes see during the day is the very same Moon that we sometimes see at night. We can only ever see the same side of the Moon, whether we see the Moon during the day or during the night. The dark regions that one can see on the Moon at night can also be seen during the day. The daytime Moon is almost equally bright as the nighttime Moon of the preceding or following night. The Moon does not seem to be very bright during the day because we then compare it with the bright blue sky, but the Moon does appear very bright at night because then the sky is very dark.

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5. If the Moon Disappeared

If the Moon disappeared then the tides would reduce in strength by about two thirds. There would still be tides, because the Sun causes tides, too (see the Conjunctions Page).

The Moon keeps the angle between the rotation axis of the Earth and the plane of the orbit of the Earth within a fairly narrow range, so that the strength of the seasons cannot differ too much from its current value. If the Moon disappeared, then the inclination of the rotation axis of the Earth would vary much more over periods of thousands of years and could get much further away from the upright position, with corresponding greater differences between summer and winter, and hence with great influence on the climate, such as Mars has experienced (without any large moon). See http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1993Natur.361..615L, http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002A%26A...384..689E and http://arxiv.org/abs/astro-ph/0112399 (for http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001EM%26P...85...61B).

I've heard of kinds of animals that synchronize some rhythms of their lives with the phases of the Moon, and the sudden disappearance of the Moon could cause trouble for them. I believe I've heard of some kinds of turtles that come to lay their eggs on the beach at a certain Full Moon, and that the nightlife of deer and similar animals takes the phase of the Moon into account. In any case, fishermen and hunters seem to take the phase of the Moon into account, but perhaps that is because they would like to be able to see something at night. I am not a biologist, fisherman, or hunter, so I do not know the details of this.

It is highly unlikely that the Moon will just disappear. Only the very close passage of a celestial object of the size of a planet could wrench the Moon from the grip of the Earth's gravity, and such an event could also change the orbit of the Earth itself markedly, so then the disappearance of the Moon might not be the most important thing. Calculations of the future of the orbits of the planets show that such an event will not happen during the coming thousands of millions of years (at least not with one of the other planets of our Solar System as the disturbing object). See http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002MNRAS.336..483I and http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1996CeMDA..64..115L.

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6. The Influence of the Moon

There are people who claim that the Moon or the phase of the Moon (full moon, new moon) has influence on all kinds of things where you would not immediately expect such influence, such as the birth dates of children, or how much luck you have, or the weather. Is that true? Sometimes these people suggest that this influence comes from special lunar forces that science doesn't know or does not want to know. Can that be true?


6.1. Direct Influence by the Moon

The only direct influence that the Moon has on Earth is through its gravity and its light. The gravity of the Moon causes part of the tides on Earth. The direct influence of the gravity of the Moon on people is much smaller than the influence of the gravity of earthly things that are much closer, such as houses and cars and other people, and that influence is already so small that you don't notice it. (See the Conjunctions Page for more information about this.) Moonlight is reflected sunlight and allows us to see better sometimes at night, though we are much less dependent on moonlight these days, with all of our streetlights and lamps in our homes.


6.2. Indirect Influence by the Moon

These direct influences yield indirect influences. Without tides, there would be no tidal areas such as the Waddenzee on the coast of the Netherlands, with the related plants and animals and tidal power stations. Many animals prefer to look for food at night when the Moon is shining. The full moon at harvest time was called the Harvest Moon because people could continue to harvest after sunset by the light of that moon. However, for all of these things it is not important that it is the Moon that provides the tides or the light, but only that there is something that provides tides or light.

If something depends directly or indirectly on the Moon, then all things that depend on that first thing can themselves start depending on the Moon. If the farmer can keep harvesting for longer by the light of the full moon, then he'll probably sleep less than usual afterwards, so he'll be more sleepy the next day than usual. That means there's a connection between the phase of the Moon and the sleepiness of the farmer, even though no special sleepiness rays come from the Moon to affect farmer's brains. That sleepy farmer may not feel like playing his usual card game with the neighbors the next day, so then there is a connection between the lunar phase and the playing of card games. The neighbors may have seen this coming, and may have decided not to bake the cake that they usually eat while playing cards, so now there's a connection between the lunar phase and the sale of cake ingredients. In this way, very indirect connections can arise between things of which you would not expect this at first sight.

The problem with such indirect connections is that it is often almost impossible to determine where they came from. If you notice that there is a connection between the lunar phase and the sale of sugar in a particular village, then you may not immediately realize the importance of the card game buddies of a farmer in harvest time, especially because one can think of many other equally indirect ways in which there may be a connection between the phase of the Moon and the sale of sugar in the village, and perhaps all of them are important.


6.3. Correlation with the Lunar Phase

If someone claims that there is a connection between something and the phase of the Moon, then you should ask the following questions:




How certain is it that the claimed correlation really exists?

No source is given for many of these stories, so it is almost impossible to determine if the story is based on solid research, or if someone just made it up.

Besides, if you compare two things that both vary with time but that have absolutely nothing to do with each other, then they'll still occasionally go up or down or appear to be connected in some other way, purely by chance. If you happen to study those things during such a period, then you could conclude that they are connected, even when they aren't.

In addition, people tend to remember or pass stories on to others much more easily if the stories are about strange or unexpected things or if the stories agree with ideas that the people already had. For example, if a hundred researchers study the connection between the color of babies' eyes and the lunar phase at their birth (to name something for which a connection seems unlikely), and if they use a test that is 99% accurate, then probably one out of the one hundred researchers will by accident get data from which he concludes that there is a connection, even if there really is no such connection and even if all researchers work accurately and honestly. If all researchers publish their results independently, then the results of the one researcher that found the strange connection are likely to get much more interest than the results of the 99 researchers that didn't find anything special. It is therefore much more likely that you'll hear about the one study that found something strange than that you'll hear about the 99 other studies that didn't find anything unusual.


How much of the variation is explained by the lunar phase?

Even if a scientifically thorough study shows that there is some correlation between something and the lunar phase, then that doesn't mean that this is at all useful in practice. For example, if 90% of the variation in temperature during a month were explained by the lunar phase, then you could make a pretty reasonable prediction of the temperatures for the coming month from the expected lunar phases during that time, with only 10% of temperature variation due to other things. If instead only 0.1% of the variation in temperature is correlated with the lunar phase, then knowing the lunar phase does not help you at all in predicting temperatures, because then 99.9% of the variation is not connected with the lunar phase.

People should always mention how much of the natural variation in the investigated thing can be explained from the lunar phase. Any found connection is only useful if you can explain a large fraction of the variation from the lunar phase.

Because of the innumerable indirect connections in nature and between human actions and affairs, it is likely that almost anything is somehow connected with the phase of the Moon, so if you can do a sufficiently big statistical investigation, then you can detect lunar influence in almost everything, but in almost all cases the influence of the lunar phase is so incredibly small that it is of no use at all in practice.




6.4. The Phase of the Moon Might Have Influence on...

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6.4.1. ... Insomnia

It is possible that people sleep worse at full moon, because of the extra light that is then around at night. It is not important that the extra light comes from the Moon; if instead a bright lamp shone into the bedroom that appeared as bright as the full moon, then you'd have the same trouble to sleep. Thicker curtains and better closing of the gaps through which light can still enter the bedroom might help.


6.5. The Phase of the Moon Can Have a Tiny Effect on...

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6.5.1. ... the Weather

Some people think that the weather is related to the phase of the Moon. For example, that there is a greater chance of frost at Full Moon than at other phases of the Moon. Any such relationships are at best very weak, because the weather can be very different from place to place, while everyone has the same phase of the Moon at the same moment, and the weather can be very different from day to day or even from hour to hour while the phase of the Moon changes only slowly. I think it is fair to say that there is no correspondence between the phase of the Moon and the weather that you could use to measurably improve weather predictions.

The weather and weather-related events (such as crop losses because of unexpected early frosts) have been recorded for a very long time already, and the phase of the Moon can be calculated for any date, so if any useful correlations exist between the phase of the Moon and some aspect of the weather, then it should not be difficult to find it. Yet, I have not heard of any such useful correlations, so I must assume that none have been found. (See below for my own research into the connection between the Moon and the temperature.)

If studies have found no correlation between the phase of the Moon and the occurrence of frost, then any direct or indirect effects of the Moon on frost must be so small as to be of only academic interest (if even that). This won't change, no matter how much we discuss potential sources of such effects. If we discovered a potential source that ought to have sufficient effect to be clearly noticeable, then we would have to conclude that there must be some other effect to counteract the first one, if the observations tell us that the net effect of all known and unknown contributions is too small to measure.

Notwithstanding the merely academic interest, we can still discuss the heat reaching the Earth from the Moon. Heat loss through thermal radiation is proportional to the fourth power of the temperature (relative to the absolute zero point of temperature; e.g., measured in kelvin) at the locations from where the radiation escapes. (This is expressed by the Stefan-Boltzmann Law, see, e.g., http://en.wikipedia.org/wiki/Stefan-Boltzmann_law.)

So, if an object attains room temperature (290 K) in sunlight, then its heat loss is proportional to 290^4 = 290*290*290*290 = 7073 million units, so its heat gain through sunlight must be similar to that number. The brightness of the Full Moon is about one part in half a million of the brightness of the Sun, so the heat gain through moonlight would be about 7073 million units divided by half a million, or about 14,000 units. Adding these 14,000 units to the 7073 million units leads to a rise in the temperature of at most about 0.00015 kelvin (because the fourth power of 290.00015 is about 14,000 units more than the fourth power of 290), which corresponds to about 0.00027 degrees Fahrenheit. I expect such an increase to be completely negligible.

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?2001EM%26P...85...99C says that any temperature effects with a roughly monthly cycle are more likely associated with solar activity (the Sun rotates in about a month) than with heating by the Moon. In other words, effects from solar activity are greater than effects from the Moon, and solar activity is not tied to the phase of the Moon.

http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999GeoRL..26.1605C says that there is on average a 0.1 degree Celsius greater daily surface temperature range (difference between maximum and minimum temperature) near full moon than near new moon, which is mostly caused by higher maximum surface temperatures near full moon, which are themselves attributed to the Earth being slightly closer to the Sun at Full Moon than at New Moon (because of the gravity of the Moon).

But once more, all of this talk is academic if no useful correlation is actually found to exist between the phase of the Moon and the occurrence of frost (or any other characteristic of the weather