c) Lunar eclipse


Moon Umbra

Fig. 7.6. (a) A total solar eclipse can be seen only inside a narrow strip; outside the zone of totality the eclipse is partial. (b) An eclipse is annular if the Moon is at apogee from where the shadow of the Moon does not reach the Earth. (c) A lunar eclipse is visible everywhere where the Moon is above the horizon

Fig. 7.7. The total eclipse of the Sun occurred in 1990 over Finland. (Photo Matti Martikainen)

to the definition, a solar eclipse is not an eclipse but an occultation!) If the whole disk of the Sun is behind the Moon, the eclipse is total (Fig. 7.7); otherwise, it is partial. If the Moon is close to its apogee, the apparent diameter of the Moon is smaller than that of the Sun, and the eclipse is annular.

A lunar eclipse is total if the Moon is entirely inside the umbral shadow of the Earth; otherwise the eclipse is partial. A partial eclipse is difficult to see with the unaided eye because the lunar magnitude remains almost unchanged. During the total phase the Moon is coloured deep red because some red light is refracted through the Earth's atmosphere.

If the orbital plane of the Moon coincided with the plane of the ecliptic, one solar and one lunar eclipse would occur every synodic month. However, the plane is tilted about 5°; therefore, at full moon, the Moon must be close to the nodes for an eclipse to occur. The angular distance of the Moon from the node must be smaller than 4.6° for a total lunar eclipse, and 10.3° for a total solar eclipse.

Two to seven eclipses occur annually. Usually eclipses take place in a set of 1-3 eclipses, separated by an interval of 173 days. In one set there can be just one solar eclipse or a succession of solar, lunar and another solar eclipse. In one year, eclipses belonging to 2 or 3 such sets can take place.

The Sun and the (ascending or descending) node of the lunar orbit are in the same direction once every 346.62 days. Nineteen such periods (= 6585.78 days = 18 years 11 days) are very close to the length of 223 synodic months. This means that the Sun-Moon configuration and the eclipses are repeated in the same order after this period. This Saros period was already known to the ancient Babylonians.

During a solar eclipse the shadow of the Moon on Earth's surface is always less than 270 km wide. The shadow moves at least 34km/min; thus the maximum duration of an eclipse is 72 minutes. The maximum duration of a lunar eclipse is 3.8 h, and the duration of the total phase is always shorter than 1.7 h.

Observations of the stellar occultations caused by the Moon formerly served as an accurate method for determining the lunar orbit. Because the Moon has no atmosphere, the star disappears abruptly in less than 1 /50 s. If a fast photometer is used for recording the event, the typical diffraction pattern can be seen. The shape of the diffraction is different for a binary star. In the first decades of radio astronomy the occultations of some radio sources were used for determining their exact positions.

The Moon moves eastwards, and stars are occulted by the dark edge of the Moon during the first quarter.

Therefore occultation is easier to observe, and photometric measurements are possible; at the same time it is much more difficult to observe the appearance of an object. There are some bright stars and planets inside the 11° wide zone where the Moon moves, but the occultation of a bright, naked-eye object is quite rare.

Occultations are also caused by planets and asteroids. Accurate predictions are complicated because such an event is visible only in a very narrow path. The Uranian rings were found during an occultation in 1977, and the shapes of some asteroids have been studied during some favourable events, timed exactly by several observers located along the predicted path.

A transit is an event in which Mercury or Venus moves across the Solar disk as seen from the Earth. A transit can occur only when the planet is close to its orbital node at the time of inferior conjunction. Transits of Mercury occur about 13 times per century; transits of Venus only twice. The next transits of Mercury are: May 9, 2016; Nov 11, 2019; Nov 13, 2032 and Nov 7, 2039. The next transits of Venus are: Jun 6, 2012; Dec 11, 2117; Dec 8, 2125 and Jun 11, 2247. In the 18th century the two transits of Venus (1761 and 1769) were used for determining the value of the astronomical unit.

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