Читать книгу Eclipse: The science and history of nature's most spectacular phenomenon - J. McEvoy P. - Страница 12

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.