Things Passing In Front Of Other Things. . . Part I

We often speak of the stars as being fixed in the heavens or refer to their motion only in vast time scales compared to our lifetimes.  But the things that truly pique our interest in astronomy involve movement.  The planets, for example, are special because they move relative to the fixed stars.  Even the word “planet” comes from the Greek word for “wanderer”.  Because things in the sky actually do move, they occasionally pass in front of other things. . . and those events can be both visually striking and provide insight into our place in the Universe.  In this article, I will focus on things that pass in front of our star, the Sun.

The most spectacular and well-known event is, of course,  the Solar Eclipse.  This occurs when the Moon passes in front of the Sun.  The Sun is 390 times further away from the Earth as the Moon but it is also about 400 times larger.  This astonishing coincidence means that both objects appear to be about the same size from our point of view.  The Moon to Earth distance varies somewhat so sometimes the Moon almost exactly covers the Sun.  This is the case for a total eclipse.  If the Moon is slightly further away from the Earth, it appears smaller and leaves a thin ring of the Sun visible.  This is called an annular eclipse. 

“Astronomers Studying an Eclipse,” a 1571 painting by Antoine Caron, oil on panel. The original title is not known

The next solar eclipse will be July 11, 2010 in the South Pacific including Easter Island.  If you have the time and the means, there are cruises heading that way specifically for the eclipse.  Otherwise, if you are willing to wait it out, there are some eclipses coming to North America.  The next eclipse near the US Midwest won’t occur for another 7 years!  Here is a map of upcoming solar eclipses in North America through 2050  Although most of the country will see at least partial eclipses, the blue shaded paths will see the fullest eclipse.

Sometimes the Earth passes in front of the Sun and puts the Moon in its shadow.  This is called a Lunar Eclipse.  These are great to see for several reasons — not the least being that they happen much more frequently!  Another reason is that you can see the “inner” and “outer” shadows of the Earth a.k.a the “umbra” and “penumbra”.  The umbra appears darker than the penumbra.  The Moon turns a dark reddish orange at the deepest part of the eclipse.  Something to watch for is the curved shadow of the Earth’s disk crossing the face of the moon.  The next Lunar eclipse visible in North America will be Dec 21, 2010.  Let’s hope for a clear night!

Harken Observatory Image — Near Totality Feb 21, 2008

Sometimes the inner planets, Mercury and Venus, pass in front of the Sun.  These events are called transits.  It is an extremely rare event for Venus to transit the Sun.  the last time this happened was June 8, 2004.  Before that, the previous transit was in December 1882!  Surprisingly, the next transit ( and the last one this century) will occur on June 6, 2012.  Mark your calendars!

  DANGER! To observe the transit, you need to follow the same precautions as used for observing the Sun! 

Past transits of Venus were used to calculate the Astronomical Unit (AU).  This is the average distance from the Sun to the Earth.  In 1677, Sir Edmund Halley (Halley’s Comet) devised a method to calculate the AU by timing the transits from widely spaced locations on the Earth and applying geometry and orbital mechanics to the problem.  This method was later refined by Joseph-Nicolas Delisle.  The solution had to wait until the next transits in 1761 and again in 1769.  The observations made then narrowed the estimate of the AU to somewhere around 95 million miles.  The currently accepted distance is 93 million miles — not bad!

Glass-plate negative photographs of the 1882 transit of Venus taken by one of the U.S. Naval Observatory expeditions
Courtesy of the U.S. Naval Observatory Library

I would like to conclude this post with an amazing picture taken by French Astrophotographer Thierry Legault (http://www.astrophoto.fr/)  Sometimes other things pass in front of the Sun — man-made things.  In this case, it is the Space Shuttle Endeavor docked with the International Space Station on July 26, 2009.  Keep in mind that that there are about 93 million miles between the ISS and the Sun. 

 Words fail me.

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In the next post, we will look at things passing in front of other stars than the Sun. 

Clear Skies,

Tim Walkowski