Eclipse: The science and history of nature's most spectacular phenomenon. J. McEvoy P.

the celestial sphere defined and understood, all aspects of the apparent motion of the Sun and Moon necessary to describe eclipses are in place. However, it should be kept in mind that though this model may be useful, it is of course not a true picture of nature. The Sun does not move around the Earth. In fact, the opposite is true. Nevertheless, the celestial sphere model shows the sky as it appears to an observer on the Earth. So it is possible to speak of the ‘Sun orbiting the Earth’ in terms of the celestial sphere. The model allows us to define the positions of celestial bodies, and greatly simplifies the visualisation of their motion. It is used by astronomers the world over to measure and report observations of Sun and Moon, planets and stars. During the eighteenth century celestial globes based on the celestial-sphere model were popular accessories among the upper classes. Definitely de rigueur in fashionable British and European salons.

      A solar eclipse happens when the Moon moves into alignment between the Sun and the Earth, casting its shadow on the Earth and blocking off the Sun’s light. Alternatively, if the Moon moves into alignment with the Sun but behind the Earth, the Earth’s shadow falls on the Moon. This is called a lunar eclipse. If the orbits of the Sun and the Moon were in the same plane, a solar eclipse would occur at every new Moon and a lunar eclipse would occur at every full Moon. This doesn’t happen because the orbit of the Moon is inclined at 5° to the orbit of the Earth around the Sun, as shown in Figure 1.1. The conditions for an eclipse to occur are that the Moon must be new or full and close to one of the nodes. If the new Moon is close to a node, a solar eclipse may occur; if the full Moon is close to a node, a lunar eclipse may occur. As we shall see, the proximity of the Moon to a node is critical. But first, we shall look more closely at the changing phase of the Moon as it moves in its orbit, and the relation of this to eclipses.

       Figure 1.2. ‘The First Lecture in Geography and Astronomy’, 1748, based on the celestial-sphere model of the sky.

      Hulton Getty, London

      THE MOON AND THE EARTH: PHASES

      One aspect of the Moon’s motion which is important for understanding eclipses is the cycle of the phases it presents to the Earth during the course of a month. This is illustrated in Figure 1.3. New Moon marks the start of the cycle, when the Moon cannot be seen because it is in the same direction as the Sun and the illuminated, sunlit side faces away from the Earth. A day later, however, the Moon has moved away from the Sun and is seen as a slim crescent in the evening sky just after sunset before disappearing over the horizon. A few days later, the waxing, growing Moon is seen higher in the sky, now increasing its angular distance from the Sun at a rate of just over 12° per day. (It moves through one complete cycle of 360° in a little over 29.5 days, a period called a synodic month.) Between the 7th and 8th days the Moon reaches first quarter, making a right angle with the Sun as seen from the Earth, and the right half of the Moon is now illuminated. Between the 8th and 15th days the illuminated portion continues to grow until midway through the cycle, when full Moon is reached.

      At full Moon the entire lunar disk is illuminated and 180° separate the Moon from the Sun. It is in this position that a lunar eclipse is possible, depending on how close the full Moon is to one of its nodes. As the month plays out, the illuminated portion decreases. The waning Moon reaches third quarter after about 22 days, when only the left half is illuminated. The Moon again makes a right angle with the Sun as seen from the Earth. The Moon will rise before the Sun, visible in the morning sky. After last quarter the Moon moves closer and closer to the Sun, showing a progressively thinner crescent. On the 27th and 28th days the Moon is seen only just before sunrise, a sliver in the morning sky as new Moon approaches. After 29.5 days the Moon passes out of view again at new Moon. If the new Moon is close enough to one of its nodes a solar eclipse will occur. After new Moon, the cycle starts all over again. Figure 1.3 shows that a lunar eclipse can occur only at full Moon, and a solar eclipse can occur only at new Moon.

       Figure 1.3. The four chief phases of the Moon and the times when eclipses can occur.

      A PENNY IN YOUR EYE: A DIY ECLIPSE

      It is easy to simulate a solar eclipse. First, take a circular drinks coaster about 10 cm in diameter in your left hand, and extend it to arm’s length. Then take a penny in your right hand. Close one eye. Now view the two disks along the same axis, and adjust the position of the penny closer and closer to your eye until it just obscures your view of the coaster. You have just simulated a total eclipse of the Sun. For the 2 cm penny and a 10 cm coaster, and assuming arm’s length to be 70 cm, the penny will be about 14 cm from your eye.

       Figure 1.4. Making your own solar eclipse.

      The simplicity of this simulated solar eclipse may suggest that a real solar eclipse is a common phenomenon. But size considerations make a solar eclipse a remarkable event. The Sun is about four hundred times larger than the Moon. But it is also about four hundred times as distant as the Moon. As a result, the Sun and Moon appear to an observer on the surface of the Earth to be almost the same size in the sky. When the Moon is new and precisely aligned with the Sun and the Earth, the two disks can overlap nearly exactly, and a solar eclipse occurs. The Moon obstructs the Suns light and casts its shadow on the Earth.

      To demonstrate how unusual the phenomenon of a solar eclipse is, a scale drawing has been made of the Sun, Moon and Earth and the distances between them. The common scale has been achieved by using the Moon’s diameter as a measuring unit, a Moon. The scaled sizes and distances are listed in Table 1A. Note that on this scale the Earth’s diameter is 3.66 Moons, the Sun’s diameter is 400 Moons, the Earth–Moon distance is 109 Moons and the Earth–Sun distance is 42,816 Moons, If we choose a scale of 1 Moon = 0.5 mm, the Moon would look like this: • ; the Earth would be 1.83 mm, as indicated in Table 1A, and look like this: . The Sun would be 200 mm in diameter.

      Table 1A. Dimensions for a scale drawing of a solar eclipse.

      Part of the Sun is drawn to scale on the inside front cover of this book. The separation between the two bodies would be 54.6 mm, as shown on page, where the Earth and the Moon and their separation are shown to scale. The Earth’s diameter is 1.83 mm to scale, and the Moon’s diameter is 0.5 mm. Why page? Because we need nearly the entire extent of this book, from the inside front cover to page, to represent on this scale the distance from the Sun to the Earth. This is with all the pages opened out like an accordion, double-sided. At this distance the image of the Sun viewed from the Earth shrinks to 0.5 mm, and is just small enough to be obstructed by the image of the Moon, which is also 0.5 mm across on this scale.

      TYPES OF SOLAR ECLIPSE

      Not all solar eclipses are alike. There are three different types, defined in terms of the shadow produced by the Moon. Like a sundial or even a large straw hat, the Moon casts a shadow in sunlight, and in the Moon’s case the shadow is long enough to reach the Earth, causing an eclipse. Most of the time the cone-shaped lunar shadow is projected out into space, past the Earth. However, when the conditions of the Moon’s phase and orbital position are right, near a node at new Moon, the shadow strikes the Earth. As shown in Figure 1.5, one of three types of solar eclipse