Solstice and lunistice

SkyCaramba weekly astronomy blog for the week ending December 17, 2011

You probably know the sun is at southern solstice on December 22nd. This year, the moon is at southern lunistice the next day. You probably haven’t heard the word lunistice before. 

Twice a year, the sun changes its north-south motion. From about December 22nd to June 20th, the sun moves northward in the sky. At its northernmost point, it appears to stop and turn around. Then it goes southward for another six months and appears to stop and turn around again at the southernmost point. The northernmost and southernmost points are called solstices. Solstice is a word put together from two Latin terms roughly meaning “the sun stands still”. 

The moon also moves northward, then southward. The moon’s northernmost or southernmost point is called a lunistice. However, most people don’t use the term. Even astronomers tend not to. Almanacs published in the northern hemisphere say things like the moon is riding high when it’s at northern lunistice and riding low when it’s at southern lunistice. 

Solstices happen as the earth makes a revolution around the sun, so there are two per year. Lunistices happen as the moon revolves around the earth, so there are two every 27 days. There will be 27 lunistices in 2012. 

The moon is almost new with this month’s southern lunistice. If you’re watching from the northern hemisphere in the days leading up to it, you’ll see a thinning crescent moon rising before the sun almost as far to the south as the sun and moon get from your location. If you’re watching from about 22 degrees south of the equator, the moon and then the sun will rise from the east. If you’re far enough south, they will be in the northeast. 

It’s easier to see the effect of the lunistice by watching the full moon. Next month’s full moon happens three days after northern lunistice. So those watching in the north will see a very high full moon. Those watching from the south will see a rather low full moon. If you watch from month to month from the same latitude, you’ll see the full moons creep southward until June 4. On that day, the full moon will be on the same day as southern lunistice. It will be rather low in the sky for those watching from the north and high in the sky for southern hemisphere observers. 

Then the process reverses. Each full moon in the second half of 2012 will be a little further north than the one before until those of November and December which will happen close to northern lunistice. 

So is a lunistice a special occasion? It doesn’t seem to be important to any cultures. The solstices are important because they help us mark the seasons. But the moon doesn’t have that kind of hold over our weather. There are those who believe the full moon’s light somehow inspires erratic behavior. But even they don’t seem to know when the moon is high or low.