In July 2009, I had a rare opportunity to travel with an HMNS sponsored tour group to the path of a solar eclipse. That eclipse occurred the morning of July 22, 2009, and was visible in Asia and the Pacific. Unfortunately, clouds marred the event as seen from our location just outside Shanghai. But since the clouds did not completely hide the eclipse, we were able to witness some of its effects.
Solar eclipses occur when the Moon passes between the Sun and the Earth and casts its shadow on the Earth. The shadow itself, called the umbra, is the region in which the Moon completely blocks the Sun. Anyone in the Moon’s umbra experiences a total eclipse of the Sun. As the Moon passes in front of the Earth, its shadow traces a path across the Earth’s surface; this is the ‘path of totality’. To see a total solar eclipse, one must travel to a place on the path of totality. As it happens, last month’s path covered parts of India, the Himalayas, China, and the open Pacific.
In an interesting coincidence, the Moon is about 400 times smaller than the Sun and about 400 times closer. Thus, the Moon and Sun appear to be about the same size (just over 1/2 degree across) in our sky. However, the Moon had been at perigee (closest approach to Earth) on July 21, making it slight larger than usual in our sky. Further, every year in early July (July 3 in 2009) the Earth is as far as possible from the Sun (called aphelion). These factors combined to make the New Moon of July 22 8% larger than the Sun in our sky. Thus, this is the longest eclipse of the 21st century, lasting 6 minutes and 39 seconds when seen on the centerline at local noon.
This was the latest eclipse in Saros cycle 136. Astronomers in ancient Babylon noticed that similar solar and lunar eclipses recurred every 18 years, 10, 11, or 12 days, and 8 hours. This corresponds to 223 lunations. (One lunation is the period from one New Moon to the next–about 29.5 days). The 10, 11, or 12 days depend on how many leap years are in the 18 year period. In 1691, Edmund Halley applied the name ‘saros’ to this cycle, based the ‘SAR,’ a Babylonian unit of measure. It turns out that the unit for keeping track of eclipses in Babylon was not the SAR, but Halley’s term stuck. Cycle 136, then includes the eclipses of July 11, 1991, June 30, 1073, and June 20, 1955. Future eclipses in this cycle will occur on August 2, 2027, August 12, 2045, and so on. As eclipses of cycle 136 occur further and further from aphelion, they won’t be quite as long as this year’s. There won’t be a longer total solar eclipse until June 13, 2132. That’s when a different saros cycle, #139, begins to occur near aphelion.
The Shanghai Tourism Administration estimates that over 13,000 overseas visitors traveled to Shanghai to watch the eclipse. Along with hundreds of other eclipse chasers, our group left Shanghai proper to observe the eclipse from the Yangshan Deep Water Port, a small island southeast of the city itself. To understand why, refer again to the July 2009 path of totality. Drawn on the eclipse path on that map is a black Sun with small rays, indicating a point on the open water southeast of Japan. This is the point of maximum eclipse, where the eclipse occurred at local noon and lasted the full 6 minutes and 39 seconds. At other places on the path, totality was slightly shorter. A few folks actually sailed the Pacific in order to be near that point. We, however, opted for the convenience of observing on land. Shanghai was the place in the path of totality closest to the point of maximum eclipse while still on the Asian mainland.
Also, note the blue line drawn down the middle of the path of totality. Observing on that line, as opposed to the northern or southern edges of the path, gives you a longer eclipse. Shanghai, although well within the path, is somewhat north of the blue centerline. Moving from Shanghai itself to Yangshan island to the southeast put us closer to the centerline. This gave us 5 minutes, 57 seconds of totality as opposed to about 5 minutes even in Shanghai.
|Photo from Shanghai, 2009 solar eclipse
As it turns out, there was another benefit from observing from Yangshan. July 22, 2009 was rainy in Shanghai. At Yangshan, however, it was simply overcast. And just when we were beginning to think we’d miss the entire event, the clouds began to thin out in spots, allowing us occasional glimpses of the partially eclipsed Sun.
Unfortunately, those thinner clouds were not with us during totality. We missed seeing the beautiful corona around the totally eclipsed Sun. We could not see the planets and the brighter stars against the mid-day twilight sky. And we could not watch the Moon’s shadow approach and then leave us making shadow bands on the ground as it did so. However, we did notice how much darker and cooler it got during totality. After all, an overcast sky at night or in twilight is much darker than an overcast sky in broad daylight. Cheers and whistles rose from Yangshan as darkness fell at 9:37 am and lasted until 9:43 am local time.
|Photo from Shanghai, 2009 solar eclipse
Literally seconds after totality was over, the clouds once again became thin enough for us to see the Sun through them. As we watched the Sun come out of eclipse, we gave thanks for having avoided the rain and for being able to see as much as we saw, although we wished the clouds had thinned a little earlier to give us a glimpse of totality.
Would you like to have a similar experience? Well, the path of the next total solar eclipse, on July 11, 2010, scarcely touches land at all, although it does pass over exotic Easter Island. On November 13, 2012, totality is visible from northern Australia.
Can’t afford to leave the country to see an eclipse? The Moon’s shadow crosses the United States on Monday, August 21, 2017. The path of totality for that eclipse passes roughly from Salem, Oregon to Charleston, South Carolina. How about a total eclipse right here in Texas? Mark April 8, 2024, on your calendars. On that date the Moon shadow first touches land near Mazatlan, Mexico, then sweeps right across the center of Texas before heading off to the northeast. Folks in Dallas, Austin, and the western part of the San Antonio area see a total eclipse on that date; Houston experiences a deep partial eclipse. The really young can look forward to May 11, 2078. On that date, the Moon’s shadow passes just south of the upper Texas coast on its way to New Orleans and Atlanta. Houstonians again experience a very deep partial eclipse.
The Moon’s shadow, then, will visit North America several times in the 21st century. Maybe you can go observe the rare and beautiful spectacle of a solar eclipse, with better luck than I had in Shanghai.