The Durations Of Eclipses

Total solar eclipses are brief. Although a small fraction last for as long as seven minutes, most present a period of totality lasting only two or three minutes. The partial phase of such an eclipse lasts for much longer, some hours.

Refer back to Figure 2-3, and imagine that you are waiting somewhere on the track that the spot of totality will eventually cross, blanking out the Sun for a couple of minutes. The radius of the footprint delineating the penumbra is about 2,000 miles, and it sweeps across the globe at around 1,600 miles per hour. This means that the partial phase starts about 75 minutes before totality is achieved and continues thereafter for a similar interval. People located well north or south of the track will see only a partial eclipse, but it may last for a couple of hours.

The specifics may be rather different for particular solar eclipses, especially for observers situated close to the edges of the planet in this view, but the broad picture is correct: totality lasts for a couple of minutes, partiality for over an hour before and after.

How long do lunar eclipses last? Since the Moon is large, it is conventional to define several distinct contact points or times, as shown in Figure 2-5. The Moon is within the penumbra between

FIGURE 2-7. A selenelion photographed over the city of Adelaide, South Australia, in January 2001. The Sun has just risen in the east, behind the photographer, and its feeble light is starting to illuminate the city, although street lamps can still be seen. The western horizon, out over the sea, can hardly be distinguished in the gloom. Less than a degree above the horizon is the Moon, in partial eclipse, meaning that both the Sun and the eclipsed Moon may be seen at the same time. This is only the twelfth time in history that such an occurrence has been recorded.

FIGURE 2-7. A selenelion photographed over the city of Adelaide, South Australia, in January 2001. The Sun has just risen in the east, behind the photographer, and its feeble light is starting to illuminate the city, although street lamps can still be seen. The western horizon, out over the sea, can hardly be distinguished in the gloom. Less than a degree above the horizon is the Moon, in partial eclipse, meaning that both the Sun and the eclipsed Moon may be seen at the same time. This is only the twelfth time in history that such an occurrence has been recorded.

P1 and P4, which lasts for up to five-and-a-half hours, during which time the Earth has executed almost a quarter of a revolution. In principle this would allow 70 percent of the planet's inhabitants a chance to see that a lunar eclipse is underway. The umbral stage is much more noticeable. The phase of totality, be tween U2 and U3, may last for 80 to 90 minutes, but can be much less if the Moon is slightly further north or south compared to the terrestrial shadow. These contact points for lunar eclipses, and the equivalents for solar eclipses, are discussed in more detail in the Appendix.

+1 0

Post a comment