Q1.
When I look at the night sky in early evening, I see some moving stars. What are they? Is their motion also periodic?
Answer
They are not stars at all — they are artificial satellites orbiting the Earth. (An aeroplane can look similar, but it shows blinking red and green lights; a satellite does not.)
How to tell a satellite apart:
- It is a point of light with a steady or slowly flickering brightness, with no colour and no blinking.
- It moves visibly and fast, crossing a large part of the sky in a few minutes, while the real stars stay fixed relative to one another.
- It is seen just after sunset or just before sunrise. That is the giveaway: it does not glow on its own but shines by reflecting sunlight, so it must still be up in sunlight while the ground below you has already gone dark.
Yes, the motion is periodic. Most of these satellites orbit about 800 km above the Earth's surface and take roughly 100 minutes to complete one orbit.
Orbits in one day = 24 × 60 min ÷ 100 min
= 1440 ÷ 100 = about 14 orbits per day
= 1440 ÷ 100 = about 14 orbits per day
Why it matters: Because the period is fixed, the passes can be calculated in advance — which is exactly what the mobile apps and websites in Activity 11.4 do when they tell you when a satellite will cross your sky. It is the same reason that made the Moon useful for a calendar: a motion that repeats predictably can be counted, and anything that can be counted can be used to keep time.
Did you know? The Moon is our natural satellite. India's artificial ones do very different jobs — Cartosat photographs the Earth for maps, city planning and disaster management, feeding the Bhuvan platform, while AstroSat looks outward at stars and other celestial objects.