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Moon
- Axis of moon makes an angle of 58 degree 43 mins to the ecliptic plane.
- Diameter: 3480 km
- Mass: 1/81 of earth
- Perigee: nearest point to earth (356000 km)
- Apogee: Farthest point to earth (407000 km)
- Period of revolution around earth: 29.53 days. This is called synodic month.
The time taken by moon to complete one rotation on its axis is exactly same as its sidereal month (27.5 days). Hence, we see the same face of the moon from the earth.
Only 59 percent of the total surface of moon is visible from earth.
Phases: The synodic month begins when the sun and the moon are in conjunction. At this time, moon is dark and called the new moon. This is followed by various phases and the month ends with again the new moon phase.
Tides:
Regular rise and fall of water level in the seas and oceans. Twice a day. Successive high tides are about 12.5 hours apart.
Tides are caused by a combined effect of the gravitational pull of moon and sun on earth. When the forces of sun and moon complement each other, higher tides known as spring tides occur. This occurs on full moon and new moon days when the earth, sun and moon are in the same straight line.
When the forces of sun and moon act contrary to each other the tides of lower magnitude known as neap tides occur. Neap tides are observed in the first and the last quarters of the moon.
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Tides are stronger at the time of perigee and weaker at the time of apogee.
- Tidal range: difference between the water levels at the time of high tide and low tide. It changes from time and place.Narrow bays have higher tidal range.
- Bay of Fundy has the highest tide range in the world.
Tides may lead to advance of water upstream into the rivers. This is known as tide bore.
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