Sky's Up July-September 2017 - Page 16

1 continued 16 being that some have a total phase. The partial phases of total eclipses differ in length at each eclipse. Places outside the path of totality only see a partial eclipse. Sometimes the center of the Moon’s shadow completely misses Earth COURTESY OF Steve Edberg and only a This January 4, 1992, annular eclipse was technically below the horizon by the time it reached the partial eclipse California coast, Black Mountain, where this image was taken. Refraction (light bending) by Earth’s is visible over atmosphere raised the Sun above the physical horizon and made the ring visible. Notice the many a limited area layers apparent in Earth’s atmosphere, presented by the variation in brightness of the ring. of our planet. But there are also solar eclipses wherein the center of the Moon’s shadow does cross our planet but the eclipse isn’t total. In this case, the Moon is too far from Earth and it does not completely cover the Sun even when it is centered on the Sun. This is a central partial eclipse, better known as an annular eclipse. During the central period (sometimes called annularity), the Moon’s silhouette is surrounded by the bright sun, making a ring (annulus) that changes shape all through the central phase as the COURTESY OF Steve Edberg silhouette moves across the Sun’s disk. The tiny silhouette of Mercury is the dot, smaller than the sunspot on Annular eclipses can last as long the left, visible on the right half of the solar disk below the equator. as about 12 minutes or be almost instantaneous. See Sidebar One on the next page for more Another phenomenon, similar to an eclipse, information on these infrequent events. is called a transit. On Earth we can observe Eclipses Seen on Other Solar System Planets transits of Mercury and Venus occasionally. With no natural satellites, Mercury and Venus The silhouettes of these two planets cross the don’t get to enjoy eclipses, though someone disk of the Sun, taking a few hours to do so. Sky ’ s Up Q1 sidebar one Phenomena related to eclipses About 13 times a century for Mercury and in pairs eight years apart separated by more than a century for Venus, we see these planets transit the Sun’s disk. The silhouette of the planet passes across the disk of the Sun, taking a few hours to make the full crossing. Mercury’s silhouette is tiny, invisible without a telescope. The silhouette of Venus is large enough that with a safe solar filter you can see it without a telescope; with a telescope Venus is a small but distinct disk. Asteroids and comets can also transit. Halley’s Comet was predicted to transit the Sun in 1910 but the passage was not visible in telescopes. This told astronomers that the solid nucleus of the comet was small. Earth satellites also transit the Sun (and Moon) on occasion. The pass takes about 1 second. The distinctive shape of the International Space Station can be seen and imaged. Sometimes spacecraft see the Moon crossing the Sun’s disk. It’s sometimes not obvious whether to call the event an annular eclipse or a transit. We call an event when a larger body passes in front of a smaller body in the sky an occultation. If we were consistent using definitions of astronomical phenomena, a total solar eclipse would be called a total occultation of the Sun. Only events during which we see the shadow of one object fall projected on another object would be called an eclipse (like an eclipse, total or partial, of the Moon when it passes through Earth’s shadow). But in common usage, an occultation refers to (usually) night time events such as the Moon passing in front of (occulting) a star or asteroid or planet. There are also occasions when an asteroid or planet occults a star. See the discussion of Jupiter in Sidebar Two for more information. COURTESY OF Steve Edberg Venus is much larger than the sunspots visible on the Sun’s disk to its right. COURTESY OF NASA/Bill Ingalls Above, this composite image made from five frames shows the International Space Station, with a crew of nine onboard, in silhouette as it transits the Sun at roughly five miles per second on Sept. 6, 2015. Left, in this view, the Moon is partially occulting Saturn. At most locations for viewing this event, Saturn eventually disappeared completely behind the Moon and reappeared an hour or less later. From this site, the Moon never completely covered Saturn and spent about five minutes sliding by, partially covering the planet. Notice that Saturn is covered by a narrow piece of the night side of the Moon, not by the mountains illuminated by Sun in the foreground. COURTESY OF Steve Edberg Sky ’ s Up 17