My Dark Sky

Solar eclipse is the most spectacular event in all of Nature. Imagine the sky briefly turns into eerie darkness in the middle of the day, and the tenuous corona that is usually invisible suddenly reveals before your eyes…

For those who have ever witnessed one, they say it is so spectacular that you have to be there personally to feel it – no word is ever sufficient to describe the feeling…

So, what is this solar eclipse that is so spectacular?

A solar eclipse happens when our Moon partially or totally blocked the Sun from our view. For the Moon to block the Sun, it has to be between us and the Sun, so solar eclipse only happens during new moon.

New moon happens every month, but not solar eclipse. Why? Because the Moon’s orbit around us is not in the same plane as our orbit around the Sun (the ecliptic) – it is tilted by 5 degrees. So, most of the time the Moon either passes above or below the ecliptic and is not in a position to block the Sun. Only when the Earth, the Moon and the Sun are in a straight line, then solar eclipse will occur.

When the Moon blocks the Sun in the sky, it actually casts its shadow on Earth. Thus, another way to define solar eclipse is that it happens when the Earth passes through the Moon’s shadow. The shadows cast by the Moon have two parts. One is the darker central region called the umbra, and another one is the lighter outer region known as penumbra.

Solar eclipse comes in three flavours depending on the distance of the Sun and the Moon from us (recall that orbits are elliptical) and their positions.

Total solar eclipse occurs when the Moon manages to completely obscure the Sun. This can happen either when the Moon is near to us so it’ll look bigger than the Sun or when the Sun is far from us so it’ll look smaller than the Moon. However, total solar eclipse only happens if you are in the umbra region. If you are in the penumbra region, you’ll only see a partial solar eclipse.

Partial solar eclipse also can occur without total solar eclipse because the umbra does not intersects the Earth. This happens when the Earth, the Moon and the Sun are not exactly in line and the Moon only partially blocks the Sun.

Annular solar eclipse occurs when the Moon is far from us or when the Sun is near to us so that the Moon’s apparent size is smaller than the Sun. The Moon cannot completely obscure the Sun and the Sun appears as a bright ring around the dark Moon.

Due to the combined motions of our own rotation and the Moon’s orbit around us, the shadows of the Moon trace out a path on Earth’s surface. This path we called it the “Path of Totality”. The width of this path is quite narrow, typically about 200 km wide or less, because the Moon itself is not that big in the sky. However, the shadows can trace a path of 15,000 km long on Earth’s surface.

The umbral shadow cast by the Moon as seen by astronaut in the International Space Station. Credit: NASA.

If you understand what have been said above, then you will know why not everyone has the opportunity to see a total solar eclipse. First of all, you have to be in that narrow path of totality. This translates to less than 1% of Earth’s entire surface area. Since 70% of our Earth surface is open seas, the chances of the Moon’s umbral shadow falls on lands is lower. Even if the shadow does traverse through lands, the odd of the shadow passing through a big city is even lower. It will usually pass by some remote or inhabitant areas.

Even if you manage to be in the path of totality, the duration of totality is not the same for all the places. Totality is the best part of a total solar eclipse – it is the moment when the Moon totally covered up the Sun. It only happens briefly, the most is 7 minutes and 40 seconds, and usually it is much much less than that. The further away you are from the Greatest Eclipse, the shorter the duration of totality.

And then you still have to put up with the weather…

Solar eclipse occurs few times a year, so it’s not uncommon. However, if we have the chance to see it, we must grab it because it may not always be accessible to us. With fewer than 70 total solar eclipses per century, the opportunity to see one may be once in a lifetime event!

I can’t finish this post without mentioning one more thing. We are really born at the right time to see total solar eclipse. The matching size of the Moon and the Sun in the sky is really a coincidence in time. The Sun is 400 times larger and yet it’s 400 times further away than the Moon. This translates to the same size of both objects in the sky.

But this is not the situation in the past, and it’ll not be like this forever. In the past, the Moon is nearer, so total solar eclipses last longer. As time goes by, the Moon is slowly moving away from us, roughly 4 cm per year. As it moves away from us, it’ll appear smaller and smaller in the sky, and eventually it’ll be too small to completely block the Sun. No more total solar eclipses then, we’ll only be left with annular solar eclipses.

~ by thChieh on January 18, 2009.

Posted in General Astro Talk
Tags: Solar Eclipse