22 March 2006


Eclipses of the Sun and Moon are special, rare events where we get to witness and take part in motions of the Solar System.

Why do we have eclipses? The path of the Sun across the sky is called the Ecliptic. The path of the Moon follows the Sun very closely but not exactly -- that is, the Moon follows the Ecliptic but is sometimes above and sometimes below it. However, during special moments every six months the path of Moon exactly intersects the Ecliptic and the result is that we experience a Lunar Eclipse at full moon (in which the Moon is darkened as it passes through the shadow of the Earth) or a Solar Eclipse at new moon (in which a part of the Earth is darkened as it passes through the shadow of the Moon), or both in a two week period.

On Wednesday March 29th the Moon will block the Sun and the shadow of the Moon will land on a broad swath of Earth from western Africa to central Asia. In this area people will observe an eclipse of the Sun. Along a narrow strip of Earth in the middle of this region the dark central shadow of the Moon (known as the umbral shadow) will create the stunning visual impact of a total eclipse of the Sun. During a total eclipse of the Sun the viewer on Earth witnesses the Moon completely covering the solar disk. This causes the sky to darken to nearly the condition of night, the temperature to drop and for the air to become still. Animals change their behavior in response to this sudden and dramatic change in the sky. For a few brief minutes in the total phase of the eclipse it is safe to look directly at the Sun because the central disk (photosphere) is covered. At this moment the faint outer regions of the Sun (corona) becomes visible. It is an awesome and beautiful experience.

Unfortunately, total solar eclipses happen infrequently and in nearly all cases you will need to travel to see one. The next total solar eclipse after March 29th won't take place until August 2008 and you will need to travel to Russia or China to see it. If you want to see one in North America you will need to wait until August 2017 and even then you will need to travel to another state to see it since it will only be visible in a line stretching from Oregon to South Carolina.

For more details on eclipses Nasa sponsors an excellent website: http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

01 March 2006

Vernal Equinox

Spring is in the air. Well, you might not realize that from the cold and the rain. But in fact you can tell by the changes in the sky. In particular, the days are getting longer and if you hadn't yet noticed, the pace of change in the amount of daylight is rapid.

The Vernal Equinox (the first day of Spring) takes place this year on March 20th. This is the day when the Sun stands directly over the Earth's equator at noon and begins its six month journey in the northern skies. From our perspective the Sun will be 52 degrees above the horizon at noon (which by the way is determined by subtracting our latitude in San Francisco from 90 degrees - a bit tricky to explain in writing so if you want to know more about this come to my lecture on March 21st!).

What does this mean for us right now? Each day the time of sunrise is arriving about 1 1/2 minutes earlier and the time of sunset is occurring about 1 minute later than the day before. So in a single week in March we experience more than 15 minutes of additional daylight compared to the week before. The Vernal Equinox is the day in which this daily change is fastest -- that is, the days are getting longer at the most rapid rate. After the Vernal Equinox the days will continue to get longer but the daily change will be slower.

All of this happens because of the tilt of the Earth on its axis. The tilt of the Earth gives us longer and shorter days and hence warmer and colder seasons. And it means that the location of the Sun, Moon and planets is not a simple, predictable line across the sky. We will look at some of the special motions of our Solar System neighbors in a coming issue about eclipses.