The Confusing Moon

Many of us look up at the moon and wonder what it is doing. One minute it is here, next minute there, one day rising on one side of the house and later seemingly has jumped to the other side of the house. It appears to scoot along the horizon day after day and sometimes it looks bigger than other times. Just as we are getting used to seeing it high above our heads, we look again and it is skipping low across the sky above the horizon.

There are sayings that mislead, sayings we have grown up with, been taught at school and are ingrained in our minds. We must first unlearn them. One is ‘full’ moon. The moon is always full but we only see it as such for one day in the month. The full moon is on the opposite side of earth to the side the sun is on. Full moon comes around every 29 days.

Seven days after full moon it is called ‘last quarter’, the "D" shape as seen from the southern hemisphere and the "C" shape if seen from the northern hemisphere. It is not a quarter of anything but is called that because each phase of the moon is seven days long, which is a quarter of the 29-day month. The month (or ‘moonth’) is the time it takes for the moon to drift once around earth.

The moon moves to the east of us each day by about 13 degrees of a 360 degree circle. If seen from space it would look almost stationary, with the earth completely rotating once per day beneath it. A time lapse camera mounted in space would show the earth spinning 29 times under the moon as the moon slowly made its way once around the earth.

Seven days after last quarter we are in ‘new’ moon phase. It isn't a new moon, it's the same old one, but the ancient Greeks thought the old moon died and was replaced with a new moon each month, hence the name. In new moon phase the moon is between earth and the sun, a daytime moon, in the sky all day but invisible against the sun's glare. It is out of the sky all night and so new moon nights are the best time to look at stars.

The next phase, seven days later is called ‘first quarter’. It is the "C" shape from the southern hemisphere and the "D" shape as seen from the northern hemisphere. The first quarter moon rises at lunchtime and sets low in the western sky around midnight. The full moon rises at dusk and sets at dawn. It is at its highest at midnight. You will never see the full moon at lunchtime. That is when the last quarter moon sets, having risen at midnight and been in the sky all night. The new moon rises at dawn and sets at dusk and is in the sky only in daylight hours.

But the moon doesn't actually rise or set because it is neither a cake nor a jelly. Moonrise and moonset would be better called earth-roll-in and earth-roll-out. And if you had a house that gave you a view of the horizon and you made sure you were there to watch the moon rise every day, you would see that these risings move back and forth along the eastern horizon 13 times a year.

If you only watched the moonrise once per month when it was at full moon, you would see that in summer the full moon rises in the northeast, in autumn in the east, in winter in the southeast and in spring in the east again. In the southern hemisphere the winter full moon is in our hemisphere and it appears to be almost overhead. Actually it is closer to being over Darwin but is far enough away that it appears roughly over us.

In the summer the full moon is over the northern hemisphere and so it is low in the sky. The full moon in summer takes the same path through the sky as the sun in winter, and vice versa - the winter full moon is where the summer sun is in the sky. It is high over us right now because it is currently over the southern hemisphere. In 14 days time it will be low on the horizon because it will be over the northern hemisphere. In a month it will be back over us again.

An old wives tale is that the moon on its back is holding water. But as we move beneath it the angle of sight is constantly changing. On a day when it looks to be on its back if you wait a few hours it will be on its side. As I look out my window at this time of writing, midnight of 14 August and 1st quarter, it is on its back and almost setting between southwest and west, but at 5pm it was sitting on its edge high in the sky between north and northeast, and yet when it rose about lunchtime it was leaning over like a banana standing on its edge..

Sometimes the full moon rising appears bigger than usual. There are two things going on here. One is the "moon illusion" in which the rising moon always appears bigger when seen on the horizon because it can be seen together with land or sea. When higher in the sky, with nothing to compare it to, it seems a smaller ball. The fault there is with human perception. But it may also be that the moon is indeed closer. Every twenty-seven days the moon to earth distance is less, called perigee. Sometimes perigee does happen on full moon day, called a supermoon, as did the May, June and July full moons. But that won't happen again until July 2014.

Ken Ring of www.predictweather.com is author of Weather Almanac for NZ 2013