Glossary

Eclipses

While we performed Activity 1 , you might remember that you had to hold the model of the Moon a little above your head or bend a little so that you do not block the light from the Sun. Does the Earth block sunlight or does it not? If it does, how do we see the full Moon?

Well, most of the time it doesn’t block the light because the Moon’s orbit is not in the plane of the Earth’s orbit. The plane of the Moon orbit makes an angle of 5° to the plane of the Earth’s orbit. Figure 3 shows an oblique view to the Sun-Earth-Moon system. The Earth is shown at position A and two snapshots of the Moon’s orbit are shown at A and B (the Earth is not shown at position B for the sake of clarity). As you can see, the orbit of the Moon makes and angle of 5° at any given point. So even when the Moon and the Sun are on the side of the Earth or on exactly opposite sides of the Earth, the angle between the Sun-Earth line and the Earth-Moon line can vary between 0° and 5°. Let us understand this in more detail with an activity.

Procedure:

1. Imagine that your head is the Earth. Fix one of the directions as the direction from which sunrays are coming.
2. Trace the orbit of the Moon around the Earth using your stretched hand. You can do it in several ways.

The orbit of the Moon can be inclined in such a way that the Sun-Earth line makes exactly a 5 ^° angle with the Earth-Moon line when the Moon and the Sun are on the same side of the Earth (as shown in position A). Here your hand will be at its highest or lowest position depending on whether the Moon is on the same side as the Sun or is on exactly the opposite side from the sun. The light will not be blocked, and there will not be any shadows. We will see the Full Moon when the Moon is on the opposite side of the Sun.

The orbit of the Moon can be inclined in such a way that the Sun, the Earth and the Moon lie on the same line, as shown at position B. Here your hand will be at the level of your head when the Moon is on the same side as the Sun or on the opposite side from the Sun, and it will be at its highest or lowest position when it is perpendicular to the direction of the sunrays. In this case, the Earth will block the light when the Moon is at the full Moon position and the shadow of the Earth will fall on the Moon.

Figure 3: The orbit of the Moon makes an angle of 5 ^° with the orbit of the Earth

At other positions of the Moon’s orbit, when the Sun and the Moon are on the same side or on exactly opposite sides of the Earth, the angle between the Sun-Earth line and the Earth-Moon line will lie between 0 ^° and 5 ^° .

Thus, most of the times the Moon passes from a little above or a little below the line joining the Sun and the Earth and, hence, is lit by the sunlight. But sometimes, the orbit is oriented such that the Sun, the Earth and the Moon are in a straight line (position B). At this time, the Earth blocks the sunlight and casts the shadow on the Moon. At these rare times, the entire Moon is in darkness. The half of the Moon which is on the other side of the Earth is also on the other side of the Sun and hence not exposed to sunlight. The half towards the Earth and the Sun is in the shadow of the Earth. This is the time of total lunar eclipse. (Luna is a Latin word which means Moon). If you get a chance to watch the lunar eclipse, do not miss it! It is a beautiful sight when the shadow of the Earth slowly slides across the Moon. Interestingly, an ancient Greek scientist, named Aristotle, proposed this as one evidence of the spherical shape of the Earth. If the Earth was of any other shape, it would not cast a spherical shadow every single time. Many people at that time believed that the Earth must be of the shape of a disk. Does the disk always cast a round shadow? Try it!

Sometimes, the angle between the Sun-Earth and Earth-Moon line is not zero but so small that the shadow of the Earth falls on only some part of the Moon. This is called partial lunar eclipse ( Figure 4 ).

Figure 4: Full Moon, total lunar eclipse and partial lunar eclipse

You might remember that while doing Activity 1 , the Moon comes in between the Sun and the Earth at the time of New Moon. Similar to Full Moon position, this does not happen very often because of the inclined orbit of the Moon. But when it does, the Moon blocks the Sun, and we see the solar eclipse. There are three types of solar eclipse (shown in Figure 5 ).

1. Total solar eclipse: Complete Sun gets blocked by the Moon

2. Partial solar eclipse: Only some portion of the Sun gets blocked by the Moon

3. Annular solar eclipse: The ring is seen where the Moon blocks only inner circle of the Sun keeping the outer ring visible

Figure 5: Solar eclipse as seen from the Earth

Figure 5a: Total solar eclipse captured in 1999 from France
(Credit: By I, Luc Viatour, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1107408)

Figure 5b: Partial solar eclipse captured on October 23, 2014 from Minneapolis, USA
(Credit: By Tomruen - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=36349192)

Figure 5c: Annular eclipse captured on May 20, 2012 from Nevada, USA
(Source: https://commons.wikimedia.org/wiki/File:Annular_Eclipse._Taken_from_Middlegate,_Nevada_on_May_20,_2012.jpg ( CC BY-SA 3.0) )

Draw a diagram which explains how solar eclipse occurs.

Use your notebook for drawing. Please write following on page where you are drawing- The Basic Astronomy Module: Unit 2: Lesson 2: Activity 2 & your Login ID