What's Up in the Sky, Jan 2011
Post date: Jan 3, 2011 9:05:47 PM
The Earth's orbit around the Sun is almost a perfect circle, but the center of that circle is not exactly at the Sun's center. Because the Earth is tilted on its axis at 23.4 degrees, when we are the closest to the Sun the northern hemisphere is pointed away from the Sun, and we that live in the northern hemisphere have winter. The Sun is low in the sky because it is now 23.4 degrees below the Equator, where everyone is enjoying summer. The Sun reached the lowest point in the sky on December twenty- first, at 5:42 PM and is now making its way north again.
Our motion around the Sun is counterclockwise, and the Earth is traveling at an average speed of 67,000 miles an hour, which means we cover the Earth to Moon distance every 3.5 hours. Every day we complete 1/365 of our journey around the Sun. This daily motion causes the stars to rise in the eastern sky three minutes and 56 seconds earlier each night. Without it we would see the same stars every night, but because of those roughly 4 minutes we see different stars in different seasons.
The apparent motion of the Sun has interested me since I decided several years ago to make a Sundial. Nothing fancy just a simple horizontal dial. I had spent a couple of years in England, and you couldn't walk through a town or village without seeing at least one sundial, usually a vertical dial on a church or other prominent building. Also the ones I saw had all been maintained though they were now more of a historical curiosity. Besides telling time, which requires correction tables, a dial can show where the sun is seasonally, give you an important date such as a birthday, or anniversary, and of course it can act as a compass. The information markings on a sundial are called furniture. The more you understand the Sun's motion, the more furniture you can put on a dial. Correction tables? The Sun's motion varies from day to day. Since clocks and watches divide time into into equal segments a correction table was made up to calculate the correct time of the Sun's shadow. This is called the equation of time. In this way sundials help us understand Kepler's second law of planetary motion: equal areas are swept out in equal intervals of time. Since the Earth moves faster when it is closer to the Sun, our sundial is fast in January as the Earth sweeps past the Sun faster in winter than it will in spring. By the Sun noon will occur a little faster each day from now until the tenth of February when a sundial will be 14 minutes and 30 seconds ahead of an accurately set watch. During the first week of November time on a sundial is a little over 16 minutes slower than a watch.
Yes, I made a sundial and it keeps local time accurately. From this model I hope someday to make a permanent dial. I also found that for what I spent on making a dial you could buy a really good watch.
Jan 02 The Earth is at perihelion, 91.4 million miles from the Sun.
07 Last quarter Moon.
15 New Moon
17 The Moon 4 deg. above Neptune 5pm CST.
18 The Moon 5 deg above Jupiter 4am CST.
20 The Moon 6 deg above Uranus 5am CST.
23 First quarter Moon.
26 Mercury at greatest western elongation 11pm CST.
27 Mars on closest approach to Earth.- 29 Mars at opposition - brighter than at any time in the last two years.
30 Full Moon., The full moon will pass 7 deg south of Mars, 2am CST.