NCERT Class 8 Science Curiosity Chapter 11 Keeping Time with the Skies PDF Download

Chapter 11: Keeping Time with the Skies

Probe and ponder

• Have you ever seen the Moon during the day? Why do you think it is sometimes visible when the Sun is up?
• Imagine you lived on the Moon instead of Earth. What would you mean by a day, a month or a year?
• What would happen if Earth had two moons instead of one? How would that change the night sky?
• If we didn't have clocks or calendars, how else could we measure time?
• Share your questions

It was Makar Sankranti, and Meera was in Ahmedabad for the Patang Mahotsav, the International Kite Festival. As she looked up at the sky filled with colourful kites, she noticed the Moon shining during the daytime. She was surprised as she had always thought the Moon appeared only at night. Also, the Moon did not appear like a full circle, but that didn't surprise her as much. She knew its shape changed every night. Still, it got her thinking. She remembered learning that the Moon is spherical and shines by reflecting sunlight. Then why isn't the whole Moon visible every night? For a moment, she wondered if it was due to a lunar eclipse. But eclipses are rare and brief. So, what causes the Moon's changing shape?

11.1 How Does the Moon's Appearance Change and Why?

Let us carefully watch the Moon to understand how its appearance changes over a month. You may have done a similar activity earlier, but let us now do it in more detail. Begin this activity from the sunrise after a full Moon day, that is when it is easiest to spot the Moon in the sky.

Activity 11.1: Let us explore

• Spot the Moon at sunrise in the western direction on the first day after the full Moon.
• Make a table similar to Table 11.1 in your notebook. Document the following:
  • Date
  • When you saw the Moon (at sunrise or sunset)?
  • Shade the corresponding Circle with pencil to show the bright portion of the Moon as shown in Fig. 11.1.
• From the second day onwards also document the following.
  • Is the size of bright portion of the Moon increasing or decreasing from the previous day.
  • Whether the Moon appears closer to or farther from the Sun in the sky than the day before.
• After about 15 days, you may not be able to see the Moon at sunrise or sunset. For the next 15 days, carry out this activity at sunset.

[Fig. 11.1: Shading the dark portion of the Moon, See in your textbook]

It is best to begin this activity a month or more before this chapter is scheduled to be learnt.

Analyse the data recorded by you in Table 11.1. Did the Moon appear different each day? Was the Moon visible on all days? Did the Moon appear at the same position in the sky as on the previous day?

Teacher's Note

When you shade the Moon's bright portion, remember you are shading only the dark part - the part you cannot see from Earth. The unshaded part is the bright portion. Many students shade it the wrong way around at first, so double-check your diagram against Fig. 11.1 before moving forward.

11.1.1 Phases of the Moon

You may have observed that the bright portion of the Moon decreases from a full circle to a half circle in about a week, as shown in Fig. 11.2. The bright portion continues to shrink for another week until it is no longer visible. This two-week period is called the waning period of the Moon. Different names are given to the Moon's visible shapes during this cycle (Fig. 11.2). The day when the Moon appears as a full bright circle is called the full Moon day (or Purnima), and the day when it is not visible is called the new Moon day (or Amavasya).

After the new Moon, its bright side grows to a half circle in about a week and to a full circle (full Moon) in another week. The period when the bright part of the Moon increases is called the waxing period of the Moon.

[Fig. 11.2: Waxing (Shukla Paksha) and waning period (Krishna Paksha) of the Moon as viewed from the Earth, See in your textbook]

Teacher's Note

The Moon does not generate its own light - it reflects sunlight from the Sun. The reason you see only part of the Moon lit up on most days is not because of a shadow falling on it, but because of the angle at which sunlight reaches the Moon from your viewpoint on Earth. As the Moon orbits Earth, this angle changes, which is why we see different phases. Think of it like this: a ball only looks partly lit when you stand to the side of a lamp, but fully lit when you stand behind the lamp.

Key Points

• The Moon appears to change shape over about 29.5 days as it orbits Earth. These different shapes are called the phases of the Moon.
• During the waxing period, the bright part of the Moon grows from nothing to a full circle over about two weeks. During the waning period, it shrinks back to nothing over the next two weeks.
• The full Moon day (Purnima) is when the Moon is completely bright, and the new Moon day (Amavasya) is when it is invisible in the sky.

This is a preview of the first 3 pages. To get the complete chapter, click below.