Science Questions

Why is the moon sometimes visible during the day?

Sun, 1st Jun 2008

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Question

Peter Williams asked:

Why is the moon sometimes visible during the day?

Answer

Weíve gone to the forum for this one.  Basically itís visible because when itís in the sky itís just another object being lit by the sun.  So when itís in the right place we can see it.

The moon is orbiting the Earth once every 28ish days.  You canít see it during the day if itís right between us and the sun because the back of it, which isnít 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.

Thereís no reason why you shouldnít be able to see it!  As long as the moonís at 45 degrees the sunward side of the Earth and itís lit reasonably youíll be able to see it.

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Peter Williams asked the Naked Scientists: Hello, This is probably a stupid question, but it's one that's bothered me for a long time: Why is the moon sometimes visible during the day, at a time when presumably it should be over the Americas? Love the podcast and thanks in advance. Peter Williams Livingston, Scotland What do you think? Peter Williams , Sat, 10th May 2008

The Moon goes around the Earth. The Earth rotates. The two periods are different (28days approx and 24 hours) At any particular time of day the Moon can be at any position, depending on the time of the month. It may be overhead or round the back of the Earth. When it's in the sky in the day, it is just another object being lit by the Sun - so we will see the parts that the Sun is shining on.
btw, when you photograph the Moon at night, the correct exposure would be about the same as for a daylit scene - it's an object in full sunlight! lyner, Sat, 10th May 2008

It is big and round and close to the earth. JimBob, Sun, 11th May 2008

A cheese in flight? lyner, Sun, 11th May 2008

here is a little flash animation I wrote a few years ago showing the moon orbiting the earth.

http://www.chaosscience.org.uk/pub/public_html//article.php?story=2004062914222354

When the moon is on the sunward side of the earth you should be able to see it during the day. Normally during the day the side of the moon pointing towards us is in shadow, so you won't be able to see it well but it is still there. daveshorts, Mon, 12th May 2008

also did you know that the moon doesn't rotate on its axis, so one side is always facing earth. caboose17, Fri, 16th May 2008

Actually, it rotates once a month!!! lyner, Fri, 16th May 2008

it rotates around the earth every month. But it doesn't rotate around its axis at all!!!!!!!!!! try reading everything i write next time!!!!! caboose17, Mon, 26th May 2008

The moon DOES rotate on it axis it takes exactly the same time to rotate as it takes to complete its orbit if it didn't we would see all of it and not just over a half as we do at the moment Soul Surfer, Mon, 26th May 2008


If it faces in a different direction throughout the month - so as to be facing us then how can it not be rotating about its axis? (Do you know what an axis is?)
Try walking around a table in the middle of the room whilst facing it all the time and without turning on an axis. lyner, Mon, 26th May 2008


If it faces in a different direction throughout the month - so as to be facing us then how can it not be rotating about its axis? (Do you know what an axis is?)
Try walking around a table in the middle of the room whilst facing it all the time and without turning on an axis.

go to this website http://www.windows.ucar.edu/tour/link=/the_universe/uts/moon1.html&edu=high
and you will feel stupid caboose17, Fri, 30th May 2008

Hello caboose17.

Thank you for taking part in our forum, and thank you for the link you have posted above which, if you read what that web page says, agrees with the other people above tried to tell you, and disagrees with you!

Here's a quote from the relevant part of the page (from www.windows.ucar.edu/tour/link=/the_universe/uts/moon1.html&edu=high, which you referred us to in your last post):

"Have you ever heard the term the 'far-side' of the Moon? Because of the effect on the Moon of tidal forces due to the Earth, the same side of the moon always faces the Earth. The rotation period and the orbital period of the Moon are the same. Therefore, Earth-bound observers can never see the 'far-side' of the Moon. Tidal forces cause many of the moons of our solar system to have this type of orbit."

What the bit in bold says is that the rotation period (the time it takes the moon to turn around once) is the same as the time it takes the moon to complete an orbit of the Earth - i.e. one month (about 28 days).

In other words, the moon is turning but it is doing so at a fraction of the speed that Earth turns, so it takes a whole month to do what the Earth does in a day. And because it is turning at this slow speed it ends up always showing us the same face, whilst the other side (the dark side of the moon) never points our way.

(Thanks for joining in, do come again, but please try note to be rude to other people in future, especially when they're right).

Chris

chris, Fri, 30th May 2008

C17
I quote from your link:
"Have you ever heard the term the 'far-side' of the Moon? Because of the effect on the Moon of tidal forces due to the Earth, the same side of the moon always faces the Earth. The rotation period and the orbital period of the Moon are the same. Therefore, Earth-bound observers can never see the 'far-side' of the Moon. Tidal forces cause many of the moons of our solar system to have this type of orbit."
Is that supposed to make me feel stupid? lyner, Fri, 30th May 2008



This is a picture taken with the TSGT during the day. turnipsock, Fri, 30th May 2008

And, if you look very carefully in one of the craters, you will spot a tiny flag, waving in the breeze.  Arf Arf. lyner, Sun, 1st Jun 2008

The Moon does not rotate around the Earth. It revolves around the Earth. Rotation = spin. Revolution = orbit. Kasey, Sun, 5th Sep 2010

What is the greatest amount of the Moon that can be seen during the day? Half? More? Kasey, Sun, 5th Sep 2010

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