Sat, 23 Jan 2010
High Noon
Tibetan New Year will be on February 14th this year. The rule of thumb is that it falls on the New Year in February and this year it's smack dab in the middle. When this happens, the Chinese and two Tibetan calendars agree on New Year's day. When it falls to either end of the month, they disagree, with the Tibetan New Year often being the later new moon.
Everyone knows that the shortest day is the Winter solstice, the first day of winter. Succeeding days are longer, but there is an asymmetry. Sunset gets later faster than sunrise gets earlier. The reason is that every day is not twenty four hours long. Some days are shorter and other days are longer and twenty four hours is only the average length. The real length of a day is measured from noon to noon, where noon is the moment when the sun reaches its highest point in the sky. Most of the motion of the sun is caused by the rotation of the earth, but a little bit of it is caused by the motion of the earth around its orbit. From our vantage point, this looks like the earth has to rotate for four extra minutes each day to catch up with the motion of the sun in the sky. This motion is not perfectly uniform, it is faster during the Northern Hemisphere's winter. This makes Winter days a little longer on average and as a result sunrise and sunset are a little later each day, independent of the length of the day. During the Northern hemisphere's Summer, days are shorter and sunrise and sunset are both a little earlier each day. But the effect is not as large and noticeable as in Winter. This effect is known as the equation of time and if you want the full explanation, check out this page on the Greenwich Observatory site. Part of the effect is due to the variation in the Earth's orbit. The Earth is closer to the Sun in the Northern Hemisphere's Winter and moves faster in its orbit. The season that the Earth is closest to the Sun is not fixed. It varies through time, mostly due to the pull of the planet Jupiter. Some believe that this variation in the Earth's orbit is responsible for the ice ages.
One of my old astronomy professors that is in the news. He headed a commision reporting on NASA's asteroid tracking program. The report says the program is behind schedule because Cangress hasn't allocated the money to support it. Which is fine by me, because if I'm going to die a firery death because of an asteroid collision, I'd rather not know about it beforehand.
