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Sunday, July 12, 2009
What happens when day turns to night?
The coming solar eclipse will be a once-in-a-lifetime event for millions of people around the world
Eclipses happen all the time. It just depends on where you are at the time. Here on earth, we've got the best ones in the solar system.
The eclipse on July 22 is a total solar eclipse, so named because the moon will totally obscure the sun's bright disk as viewed from here on earth. It will the longest one of this century, not to be surpassed until 2132.
Solar eclipses can only occur during a new moon, when the perpetually dark side of earth's satellite (as seen from earth) is facing us, and also only on those rare occasions when the sun, moon and earth's orbits are in the proper alignment at appropriate distances. This happens on average slightly more than twice a year.
It's all about geometry and shadows. The sun is 400 times bigger than the moon, but the moon is 400 times closer to us. So from our perspective, they can appear to be the same size — a phenomenon that makes Earth the only known planet with total solar eclipses. Mars has more, up to two per Martian day, but its larger of two moons can only blot out half of the sun's disk, and then only for 30 to 40 seconds.
With the sun bright in its face, the moon casts behind it a shadow. The dark shadow narrowing into a cone directly behind the moon is called the umbra. The lighter area of shadow outside that is the penumbra.
If you are within the umbral shadow, day will temporarily turn into night as all sunlight is blocked, leaving only the corona. Outside this area of totality in the penumbra, the sun will be partially blocked, less so the farther away that you get. Eventually, the sunlight angle is such that none of the shadow reaches earth.
On July 22, the 200-km wide umbra for this eclipse will fall first upon India's western state of Gujarat. It will be a sunrise with a sun taken hostage in Surat, the state's second-largest city.
In the 40-odd minutes leading up to the 6:08 a.m. daybreak, the moon will creep up on the rising sun, totally occulting it in 10 minutes. A new sunrise will start a couple minutes later, at 6:24 a.m., and take nearly an hour to come completely into view.
From there, the umbra will trace a 16,000-km-long path of totality — that cone of total darkness — arcing eastward across India to Nepal, Bangladesh, Bhutan, Myanmar and China before heading out into the Pacific, last stop Kiribati.
The 5 hour and 9 minute transit will obscure the sun by 50 percent or more for more than 2 billion people.
Although the hype surrounding this event has been about the maximum duration of totality — 6 minutes and 39 seconds — relatively few people will be in total darkness for longer than 6 minutes.
In Surat, the totality will last for 3 minutes, 14 seconds. The duration increases to 5 minutes, 57 seconds by the time it reaches Yang Shan Island, southeast of Shanghai. The Iwo Jima islands are the closest to the maximum, at 6 minutes, 36 seconds.
Outside the totality path, partial eclipses will go on much longer within the much broader path of the moon's penumbral shadow.
In Okinawa, which will be 91 percent occulted, the partial eclipse will last 3 hours and 48 minutes (from 9:32 a.m. to 12:20 p.m.). Kagoshima, on the southern tip of Kyushu, which is about 150 km closer to the path of totality, will be occulted by 96.7 percent, but its partial eclipse will be 4 minutes shorter (9:37 a.m. to 12:21 p.m.). In the major population centers of Osaka and Tokyo, the skies will go dim from about 9:15 a.m. to 12:20 p.m.
With every solar eclipse must come a warning about folks not looking directly at the corona without proper eye protection. Resist the urge. Don't look. And definitely do not look directly at the eclipse using binoculars or even through a camera viewfinder. No sunglasses either, unless they are certified welders' glasses.
A crude pinhole camera can be fashioned from two white cards: Punch a tiny hole in one and — with your back to the sun — aim and hold it so that sunlight falls through the hole onto the second card, which serves as a screen on which is displayed the inverted image of the sun.
More information can be found by visiting the NASA eclipse Web site at eclipse.gsfc.nasa.gov/eclipse.html