6degrees Astroblog
Standing in the Shadow of the Moon
By Irwin Horowitz, 8-01-08
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It is quite probably the most visually spectacular sight we can witness in the sky. In ancient times it was a source of fear as people believed that the gods were somehow angered. One of these events even ended an ancient conflict in mid-battle! It is, of course, a total solar eclipse.
There is nothing quite like standing in the shadow of the Moon and looking up to see this black hole in the sky where the Sun should be located. Surrounding this frightening visage is a pearly white glow of pale fire, which can take on many different shapes depending on how active the Sun is at the time of the eclipse.
A total solar eclipse was observed earlier today, in a swath across the surface of the Earth that began in the far northern reaches of the Canadian arctic, cut across the top of Greenland and swooped down through Siberia and along the China/Mongolia border. At its peak, totality lasted nearly 2½ minutes as viewed from northern Siberia, while those in China got to observe the setting Sun as the Moon passed in front of it.
I have had the opportunity to observe some eclipses in the past and I can state in all certainty that totality is the single most awe-inspiring sight I have ever witnessed. In 1991, I travelled with a group of friends from Los Angeles down to the southern tip of Baja California to observe an eclipse that latest nearly seven minutes. A videotape that I shot during totality on 11 July clearly demonstrates just how overwhelmed my companions and I were during the experience.
In August, 1999 there was another eclipse visible from Europe. I spent the previous twelve months planning a two month expedition to visit the entire continent with the highlight on eclipse day where I would be in Bucharest, Romania, a city that was right on the centerline for the event. Unfortunately, I had a minor problem in Istanbul a few days prior to the eclipse and was unable to get to my location. As a result, I only observed a 94% eclipsed Sun that day. I can assure you that there is a world of difference between totality and even a tiny fraction of visible sunlight. However, all was not lost as I did get to visit most of Europe that summer and came away with many new experiences.
Why do Total Solar Eclipses Occur?
There is by no means any reason we should be privileged to live during a time when these events are observable. The basic fact is that the Sun and the Moon are both approximately the same angular size in our sky. The reason for this is that even though the Sun is about 400x larger than our Moon, it is also about 400x further away. In fact, since the Moon’s orbital distance is gradually increasing over time, there will be a day in the distant future when total solar eclipses will no longer occur.
In order for a total eclipse to occur, the Moon must be located between the Earth and the Sun, in other words at New Moon. New Moon occurs every 29.53 days. However, we only get solar eclipses once every five or six months. Why don’t we see a solar eclipse every month at New Moon?
The reason for this is that the orbit of the Moon is tilted about 5° relative to the ecliptic plane (the plane defined by the orbit of the Earth around the Sun). However, the two bodies subtend an angle of about ½° on the sky. As a result, most months at New Moon the Moon passes either above or below the Sun in the sky. It is only when the Sun is located close to an orbital node of the Moon that an eclipse can occur. These nodes are the points in the sky where the Moon’s orbit around the Earth crosses the ecliptic plane.
Three Different Types of Eclipses
There are actually three different types of solar eclipses. A total solar eclipse occurs when the Moon completely covers the Sun for at least some portion of its track across the Earth’s surface. An annular solar eclipse occurs when the Moon passes in front of the Sun but is not large enough to fully cover the entire solar disk, leaving a ring or annulus of sunlight visible. A partial solar eclipse occurs when the Moon only blocks part of the Sun. All three types of eclipses have regions where a partial eclipse is visible surrounding the path of totality or annularity. However, if an eclipse is strictly a partial eclipse, it must occur in the arctic regions on Earth.
How do we get an annular eclipse? As the Moon orbits our planet, its distance will vary by more than 10%. When it is closest to Earth (perigee), its apparent size on the sky is greatest and a solar eclipse at those times will be total. When it is furthest away (apogee), it appears smallest in the sky and is not large enough to fully cover the Sun. Therefore, solar eclipses that occur at those times will be annular eclipses.
In order to get the longest possible totality, you want a big Moon (near perigee) and a small Sun, which occurs when the Earth is near aphelion (currently around 04 July each year). The theoretical maximum length of totality is about 7½ minutes. In the 20th century, we had three total solar eclipses that lasted over seven minutes at their greatest. These occurred in 1937, 1955 and 1973. In 1901, 1919 and 1991, we had eclipses that lasted nearly seven minutes.
You may notice that there is an eighteen year interval between each of these events. That is because each of these eclipses are related to each other in what is known as the Saros cycle. The Saros cycle represents a confluence of three separate lunar orbital cycles which last roughly 6585.3 days or 18 years, 11.3 days.
Based on this cycle, we know that the next eclipse should occur in slightly under one year, on 22 July 2009. Its maximum totality will last about 6 minutes 39 seconds and it will be visible in China and the Western Pacific.
For those thinking ahead, you may have noticed that the next eclipse in the Saros cycle following the 1999 European eclipse will occur on 21 Aug 2017. It will pass directly over the continental United States. Indeed, here in Idaho, the centerline of totality will pass about 60 miles north of Boise. I cannot recommend this strongly enough: if you are anywhere near the path of totality on that day, go there! Do not miss out on this opportunity. These events are rare and spectacular and you owe it to yourself to do whatever you need to do to stand in the shadow of the Moon and be a witness to this eclipse.
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The evening sky in August is dominated by brilliant Jupiter low in the south. It is easily the brightest object visible other than the Moon. Views through a telescope clearly reveal the bands of clouds in the planet’s atmosphere as well as the four large bright Galilean satellites Io, Europa, Ganymede and Callisto.
Venus returns to the evening sky this month making a close pass by both Mercury and Saturn in mid-month. They won’t be easy to spot unless you have a clear view of the western horizon after sunset.
The year’s best meteor shower will peak on the morning of August 12th. The annual Perseids are well observed due to the pleasant weather we encounter when they streak across the sky. A waxing gibbous Moon will be visible early in the evening but will set by 1:30 a.m. MDT, so you may want to wake up in the early morning hours to best observe this event.
The Boise Astronomical Society will hold their monthly membership meeting on Friday, 08 August at 7 p.m. in Classroom #2 of the Discovery Center of Idaho. Leif Edmundson will be discussing how to observe deep-sky objects like nebulae and galaxies. All are welcome to join us.
There are a number of star parties occurring in the region over the next several weeks. The best known of which is the annual Oregon Star Party, which takes place Labor Day weekend (27 Aug – 31 Aug). It is located in the heart of central Oregon, far away from any city lights.
The following weekend (05-07 Sep), the Idaho Star Party will take place at Bruneau Dunes State Park. I am pleased to announce that we have confirmed Barbara Morgan to be our featured speaker on Friday night. However, only registered star party attendees will be allowed in to her talk. On Saturday, Chris Anderson from the Centennial Observatory in Twin Falls will give a presentation on the 100th anniversary of the Tunguska event in Siberia.
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