Satellites and quick revision

4.1 Artificial satellites

The Moon is the Earth's natural satellite. Besides it, many countries have placed man-made satellites in orbit around the Earth. They show up as small moving points of light at night. Most circle at around 800 km altitude and finish an orbit in about 100 minutes.

Satellites serve weather watching, communication, navigation, disaster response and scientific research. ISRO, the Indian Space Research Organisation, operates many of them.

Cartosat sends back detailed Earth images used in mapping, urban planning and disaster response; the Bhuvan platform turns them into maps of terrain, soil, land use and vegetation. AstroSat studies stars and other celestial objects. Chandrayaan 1, 2 and 3 went to the Moon, Aditya L1 watches the Sun and Mangalyaan reached Mars. Students have also built and launched small satellites, including Jugnu, AzaadiSat and InspireSat-1, with ISRO's help.

4.2 Seeing a satellite

Spotting a satellite is a night-sky activity. Go to a place with a clear view of the sky, without tall trees or buildings, just before sunrise or after sunset, and take an adult along. Look for a point of light moving fast across the sky; its brightness may be steady or flickering. Satellites can be seen with the naked eye or with binoculars. Apps and websites list when satellites will pass over a given location.

4.3 Space debris

Satellites that stop working, and spent rocket stages, turn into space junk. Such debris clutters orbit and may hit functioning satellites. Small fragments burn up on entering the atmosphere, while larger ones can reach the ground. Nations are cooperating to clear this dangerous debris.

Vikram Ambalal Sarabhai earned the title Father of the Indian space programme by leading the push to put India's first satellites in orbit. The Vikram Sarabhai Space Centre in Thiruvananthapuram, which builds rockets and launch vehicles, carries his name.

4.4 The Moon and the tides

Near the sea, water levels rise and fall in a regular pattern called tides. A high tide returns roughly 50 minutes later on the following day. Tides track the Moon's position and phase; the lunar rise time shifts by about the same interval.

4.5 Common mistakes to avoid

• Thinking the Moon is visible only at night: it can be seen in daylight, especially near the new and quarter phases.
• Explaining phases by the Earth's shadow: phases come from seeing different portions of the Moon's sunlit half; eclipses need special alignment.
• Believing the Moon rises at the same time every day: it rises about 50 minutes later each day.
• Treating a lunar year as equal to a solar year: 12 lunar months add up to about 354 days, nearly 11 days short.
• Ignoring the extra quarter day: without leap years, the calendar would slowly drift out of step with the seasons.
• Calling an artificial satellite a star: satellites move fast across the sky, while stars appear fixed relative to one another during the night.
• Believing eclipses happen every month: the Moon's orbit is tilted, so the alignments needed for eclipses are not repeated monthly.
• Forgetting that solar calendars need correction: century leap-year rules keep the Gregorian calendar matched to the seasons.