Eclipse Week Post 1: Just what is an Eclipse?Posted: July 20, 2009
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Harbingers of death and destruction? A neat way to test how gravity works? A good opportunity to wear silly glasses and crowd around strange looking devices? Yes eclipses are all of these things and more… apart from the harbinger of death thing I just stuck that in there to scare you.
This week India will be treated to a spectacle which will see people flocking from around the world to witness. Taregna a small town in the Indian state of Bihar will be the epicenter for this event seeing the Sun completely covered by the moon. In honor of this event we here at weareallinthegutter have decided to have an eclipse week celebrating all astronomical based shadows.
To start with I want to talk about what eclipses are, how they occur and how we predict when the next one will be. At this point I feel like I have to make a confession: to a professional astronomer there is nothing more terrifying than being asked a question about what actually goes on in the sky. Ask most of us to point a constellation or why planets move the way they do and you will get an answer which will be right eventually, but to get there you will experience a lot of hand waving and muddled sentences. It turns out when you study objects many many times larger than our galaxy which contains all the stuff you normally see in the sky it’s not really that useful to know in depth details of the motion of our little rock and its closest buddies.
With that disclaimer out of the way lets get in to the heart of the matter: an eclipse is the mother of all shadows. When the sun and the moon happen to be located in the same place in the sky the light from the sun is blocked ether in part (a partial eclipse) or fully (a total eclipse) by the moon. To get an eclipse we need to have an alignment of the sun and moon and so their paths have to cross in the sky. The path of the sun across the sky is called the ecliptic: a line which passes through all the signs of the zodiac (and in no way determines how lucky you will be in love or business thank you very much!). So why don’t we get an eclipse every month? Every time there is a new moon the Sun and Moon are very close to each other, so why is it that the vast majority of new Moons dont have an accompanying eclipse? Well it turns out that the moon does not travel along the ecliptic like the Sun does but rather a line which is slightly inclined to the ecliptic by about 5 degrees or so. If they lay along the same line then eclipses would be much more regular but as it stands we get at max about 2 a year. This small inclination means that the Moon is normally a little way above or bellow the Sun. The only time we get an eclipse then is when a new moon happens to occur at the point where the path of the Sun and the Moon also cross. Following me cause I am not sure if I am at this point.
Ok so now things get complicated as the orientation of the tilt in the moons orbit also changes over time. The Sun’s gravity pulls on the moon a little bit in its orbit and the orbit changing its orientation. Its sort of like what a penny does at the end of a good spin. As it slows down it does that little funny dance where the way the face of the penny is pointing moves around. The technical word it precession but that’s not too important, what is important is that it changes the two points at which the moon’s path crosses the ecliptic and so affects when we can have an eclipse.
The third and final effect is that the moon’s orbit is not completely circular but rather elliptical which means that at some points in its orbit the moon is further from the earth than at others and so will appear smaller in the sky. Obviously this has an affect on the type of eclipse we could see. If the moon is just about the right distance from us it fully covers the sun giving a total eclipse but if it is that little further away from us then it leaves a small ring of light around the outside of the Sun which we call an annular eclipse. Simple right? Only the point on its orbit when the moon is the furthest from us also changes with time which we also have to factor into our model.
So for a total eclipse like the one this week we need to have three different cycles all matching up at the same time, we need it to be a new moon, we need that new moon to occur when the paths of the Sun and the Moon are crossing and we need the Moon to be at the right point on its orbit so that its not too far away from us. You begin to understand why events such as this are so rare and attract such large crowds
Given the fact that there are three cycles involved which after different periods of time which eventually repeat, it is interesting to ask if the pattern of eclipses also repeats. Well we have three cycles to consider: the times between new Moons which is roughly every 29.53 days, the time between the moon passing the same point on the ecliptic which happens every 27.21 days and finally the time between the the moon being at the same distance from the earth must be the same which happens every 27.55 days. To get an identical eclipse we have to have a length of time in which a whole number of each of these periods will fit. It turns out that happens only after 18.031 years which is known as a Saros cycle. This basically means that 18.031 years after this weeks eclipse another eclipse will happen which looks pretty much identical in the sky. The only difference will be that it will be visible from a different part of the Earth. In fact it has been estimated that it takes 370 years for an eclipse to occur at the same place on Earth.
As you can imagine this makes the event in Taregna all the more special for its residents as it is truly a once in many lifetimes event which they are witnessing. I for one wish I was there to see it.
Phew ok I think I got through most of that. If any of it is still unclear (which I am sure it is) the best procedure to find out more is to find your local amateur astronomy society, find out which pub they frequent, go there on a Friday night buy a round and casually bring up the topic of eclipses. In no time at all beer glasses, salt shakers and beer mats will be transformed in to a mini solar system to help you visualise what is going on. By the end of the night you will ether be an expert on eclipses or be so drunk that you don’t really care any more so its win win really.
Join us tomorrow when Emma will be talking more about why eclipses look the way they do and why Earth is lucky to have total eclipses at all.