Lesson 11 of 13
The big picture
Show on a big screenA day, a month and a year come from three sky cycles.
| Idea | Remember | Example |
|---|---|---|
| A day | The Earth spins. About 24 hours. | Sunrise to sunrise |
| A shadow clock | Shortest shadow when the Sun is highest. | A stick in the ground |
| A month | Moon phases repeat in about 29.5 days. | New Moon to new Moon |
| A year | The seasons repeat in about 365¼ days. | Summer to summer |
| They do not divide | 12 Moon months are 354 days. | 11 days short |
| A spare day every few years | A leap day about every four years. Not 1900 or 2100. | 29 February |
| Calendars | Lunar, solar or luni-solar. | Diwali moves |
| Finding the Moon | At sunset, look for a waxing Moon. At sunrise, look for a waning Moon. | A crescent low in the west after sunset |
| Satellites | Artificial satellites are human-made and orbit the Earth. They help with research, weather, communication, disasters and navigation. A satellite 800 km up takes about 100 minutes an orbit. Higher ones take longer. | Cartosat, AstroSat |
Read Chapter 11, Keeping Time with the Skies, in your NCERT book. Name the cycle first each time.