Many questions we get asked are not about the 害羞草研究所渇abric of space,害羞草研究所� the 害羞草研究所淏ig Bang,害羞草研究所� or 害羞草研究所渂lack holes,害羞草研究所� they are often questions like: 害羞草研究所淗ow do the phases of the moon work?害羞草研究所� and 害羞草研究所淚f the moon always keeps the same face towards us, does it rotate?害羞草研究所� and 害羞草研究所淲hy is the full moon high in the sky in the winter and low in the sky in summer?害羞草研究所�

Here goes.

The moon orbits the Earth, taking about 27 days to complete each revolution. During each trip it goes through its phases of 害羞草研究所渘ew,害羞草研究所� 害羞草研究所渇irst quarter,害羞草研究所� 害羞草研究所渇ull,害羞草研究所� 害羞草研究所渓ast quarter害羞草研究所� and then 害羞草研究所渘ew害羞草研究所� once more. The part of the cycle where the illuminated part of the disc is getting bigger, approaching full, is traditionally termed 害羞草研究所渨axing,害羞草研究所� and after full moon, where the illuminated portion is shrinking, the moon is 害羞草研究所渨aning.害羞草研究所� Through the whole cycle the familiar 害羞草研究所淢an in the moon害羞草研究所� is always looking in our direction. Try this backyard analogy.

Grab an old tennis ball. With a black felt tip put a happy face on one side, representing the 害羞草研究所淢an in the moon,害羞草研究所� Go into a dark room or out in the backyard at night. Take an assistant with a bright flashlight. Get him or her to stand a few metres away, shining the light in your direction; ask them to keep their eyes on the ball.

Hold the ball at arm害羞草研究所檚 length so that you see the side bearing the happy face. Start with the ball being in the same direction as the flashlight. You are looking at the dark side of the ball. If you have a dark background, the ball will be invisible. This is analogous to the new moon, where the sun is lighting up the side of the moon we can害羞草研究所檛 see.

For us in the Northern Hemisphere, the moon orbits the Earth in a counter-clockwise direction, that is, leftwards. So, keeping that ball at arm害羞草研究所檚 length, slowly turn to the left. You will see a thin crescent of the illuminated side of the ball come into view on the right side of the ball, like a letter 害羞草研究所淒.害羞草研究所� As you turn, the crescent will get broader and broader, until when you are holding the ball at right angles to the direction of the flashlight, the side of the ball facing you will be half lit up: the right hand side lit and the left side dark. This is equivalent to the phase of the moon called 害羞草研究所渇irst quarter.害羞草研究所� Keep moving. When you are facing directly away from the light, you will be seeing the illuminated face of the ball. The 害羞草研究所渕oon害羞草研究所� is now full. Then, as you continue moving, a dark crescent of the unlit side of the ball will appear on the left-hand side which will broaden until when you are again at right angles to the direction of the light. Once again half the visible side of the ball is lit, except this time it is the left hand side. We are now at 害羞草研究所渓ast quarter.害羞草研究所� As you continue to move, the amount of visible disc you can see will get narrower and narrower, looking like a letter 害羞草研究所淐,害羞草研究所� until once again you are holding the ball in the direction of the light, and you are seeing only the dark side. The 害羞草研究所渕oon害羞草研究所� is new once again. It is always easy to tell if the moon is waxing or waning because if the crescent is on the right, like a letter 害羞草研究所淒,害羞草研究所� it is waxing; if it is waning the crescent looks like a letter 害羞草研究所淐.害羞草研究所� Remember 害羞草研究所淒OC,害羞草研究所� where the 害羞草研究所淥害羞草研究所� represents the full moon.

While you were turning, the 害羞草研究所淢an in the moon害羞草研究所� faced you all the time. However, if you ask your friend with the light, you will be told that he or she saw all sides of the ball as it moved in its orbit. From the point of the 害羞草研究所渙utside universe害羞草研究所� that ball definitely turned 害羞草研究所� once per orbit. Even though you might not have seen the ball turn, you were turning, and holding the ball, so it was turning too.

When the moon is full, the Earth lies between it and the Sun, so we see the moon from the direction the light is coming from. In our winter, the Northern Hemisphere is leaning away from the sun, and towards the full moon, so it looks higher. In summer, we are leaning towards the sun and away from the full moon, so it害羞草研究所檚 lower in the sky.

Jupiter lies the south after sunset, setting in the early hours. Saturn around 11 p.m. Venus rises about 4 a.m. The moon will be new on May 25.

Ken Tapping is an astronomer with the NRC害羞草研究所檚 Dominion Radio Astrophysical Observatory, Penticton.