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Astronomy FAQ 001

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How can you tell when the Moon is waxing or waning?
And why is it called wax or wane?

Let us start with the easy bit. Wax and wane are old words for get bigger and get smaller. Note the spelling of "wane" rather than "wain" (which is a sort of wagon). To tell which is which; if the bright bit is on the right, it is waxing; if it is on the left, it is waning.

Why is this and why do we gets phases of the moon?

M01t.gif 83x146 Think of it this way. Firstly, remove the rotation of the earth. Do this by considering the same time every day, let us take just before Sun set. As the days go by, the Moon, due to its orbit around the Earth appears in a different position each day. Let us start with it in front of the Sun, which it was from Southend-on-Sea on September the 27th, 2000. M02t.gif 130x82 It will actually be below it or above it and not directly in front except at the special time of an eclipse. As it is in front of the Sun it is lit from behind and the face is only lit by earth shine, a new moon. Each day you will see that the moon, at that same time of day, is further to the left (it is lagging behind the sun in its apparent motion). As this happens the side nearest to the Sun starts to get illuminated.

M03t.gif 166x70 This is the new moon, as it is depicted in books; like a C backwards but tilted slightly downwards, as the moon is higher in the sky than the sun. M04t.gif 353x99 As the days progress, the Moon is seen further back across the sky. Each day it is roughly thirteen degrees further east. This is due to the lunar month being twenty eight days by which time we are back to a new moon again.  Note that 360 ÷ 28 ~= 13.
M05t.gif 189x99 When you get to the point that the Moon is due South as the Sun sets you have first quarter and a week has gone by since we started. Now, as the Moon is South and the Sun west, forming a right angle, the moon is illuminated from its right hand (to us) side and so that side will be bright. Notice how low down in the sky the Moon is, just barely above the horizon. Remembering that any astronomical object is at its highest elevation when due south, i.e. crossing the meridian. M06t.gif 184x86 Seven days later the Moon will just be rising as the sun sets. Since the Sun is directly behind us (if we face the Moon) and the Moon in front, the Moon will be illuminated full face, a full Moon. Of course you don't only see the moon at sunset time, but throughout the twenty four hours, well half of them. We just used the trick of considering the position at the same time each day as a means of stopping the effect of the rotation of the Earth. Actually, it should not be twenty-four hours exactly as there is a difference of approximately 4 minutes between Sidereal Time (when the stars come back to the same position) and Solar Time (when the Sun comes back to the same position), but that's another story.
M07t.gif 137x74 To observe things further we have a problem, the Sun sets before the Moon comes up. The Moon is only observable during the night when we can't see the Sun. However, on the day of the Full Moon, it is overhead (to the South) at midnight and is setting just as the Sun comes up, so we can switch to observing at dawn instead. In fact, if I plotted it exactly twelve hours later it would be below the horizon. So I have done it a little earlier so we can still see M08t.gif 162x82 it. The sky is a bit darker as it is just before dawn. As each day passes the Moon will be a little further to the left in the sky and will be illuminated less, with the bright part being on the left nearest the Sun. As the Sun is moving, relatively, faster than the Moon, it is catching it up and so will be nearer on the left (< 180°) than on the right (> 180°). Three weeks after we started the moon would be in the south at dawn and it will be third quarter. Notice how high in the sky the moon now is, as compared to what it was at first quarter. This is due to the inclination of the orbital planes and the tilt of the Earth. M01t.gif 146x83 That also is another story.   Just before the twenty-eight days are up, the moon will be back with the Sun and will be a crescent Moon again but shaped like the letter C the right way round this time, as it was when we started. On the twenty-eighth day the Moon will cross in front of the Sun again and the whole business will start another cycle.
altview.gif 552x239 An alternative way to think about this is to observe where the Sun is when the Moon crosses the meridian (the north/south line in the sky). The Sun will approach it from the left, go behind it and leave it to the right. As it does so you will go from third quarter old moon, to new moon, to first quarter. During this time the Sun will go from dawn to sunset.

This analysis is based on a simplification of the motions involved. The solar day is roughly 4 minutes different to the sidereal (star time) day, we have just ignored this by working with the solar day and not referring to the stars. However it is the stars that are fixed and everything else is moving relative to them. We have ignored other things such as the seasonal variations as the Earth moves in its orbit around the Sun, the tilt of the orbit planes relative to each other and any wobble in them, etc. The effect of these are small from one month to the next and it is necessary to understand the main concepts first before going on to add in the details.

If one wants to study things further it is a good idea to get a planetarium type program. These can usually be set so that you can view an astronomical object (i.e. the Sun or the Moon) as stationary and run an animation of the movement of every thing else.

Links for planetarium programs:-

These diagrams were produced with Sky Map Pro V5.0.16 for the location 51° 0´ N, 0° 54´ E.
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