Earth’s Moon

• Moon is the alone natural satellite of Earth. Several planets in the solar system have more than one natural satellite. Rest all planets of Solar System except Mercury and Venus have more than one Satellite. Even Mars have two natural Satellites namely Phobos and Deimos.
• Formation of Moon: There are various theories regarding formation of Earth’s natural satellite moon. Few scientists urge that Earth captured it by his gravitational pull and few others believes that Moon was formed due to a giant collision of the Earth which knocks out material from the Earth. Since Earth and Moon have similar composition so large impact theory dominates in the scientist’s community about the formation of Moon.
• Moon occupies the position of being fifth largest natural satellite in the Solar System.
• Mons Huygens is the tallest mountain on the surface of the moon which is more than half of the height of Mt Everest i.e., 18046 ft. It should be noted that Mons Huygens is the tallest mountain on the Moon but the highest point on the Moon is Selenean Summit which is 35387 ft above the centre of the Moon.
• Moon is the brightest object in the night sky.
• Distance of the Moon from the Earth is 84 × 10⁵ km and its mean diameter is 3480 km. Size of Moon is 1/4^th of the Earth and its mass is about 1/80^th of the Earth. Gravitation on Moon is 1/6^th of the gravitation on Earth.
• Moon revolves around the Earth in a elliptical orbit and makes one complete revolution around the Earth in 27.3 days. Moon also makes on complete rotation on its axis in 27 days.
• Since period of revolution and rotation of Moon is almost same, only one side of the Moon is visible from the Earth which is 59 % of the total moon surface.
• Moon emits electromagnetic waves in the radio wave region; hence it is called radio Moon.
• Temperature on Moon varies drastically during day and night. During day time its temperature rises up to 100° C and in Night it temperature falls to -150° C.
• This odd fact is due to absence of atmosphere on the Moon. As Earth’s atmosphere reflects a large portion of Sun’s light back in the space and also absorbs Sun’s heat during day time to keep the Earth moderate during night.
• No possibility of life on moon due to absence of air and water.
• Axis of the moon is having an inclination of 58°43’ with respect to the plane of ecliptic. Moreover its orbital plane is also having a tilt of about 5.15° from the orbital plane of the Earth. Means orbital plane of Earth around the Sun are not same with the orbital plane of Moon around the Earth.
• This tilt of Moon is the main reason of not having total lunar and total Solar eclipse every month.
• Phases of the Moon: The different shape of the visible or bright part of the Moon in a month is called phases of the moon.
• In the figure below different phases of the moon are depicted.
• Full moon day: The day on which whole moon is visible is called full moon day. In the figure below we can clearly see that during full moon day earth lies in the middle of Moon and the Sun. This position is not the eclipse position. We will discuss it later on which position causes lunar or solar eclipses.
• New Moon Day: After full moon day we can easily observe that that the visible or bright part of the moon are becoming thinner and thinner and on fifteenth day after full moon day it disappears this day is called as new moon day or Amawasya. After new moon day moon starts appearing in a very thin bright shape. This position is called Crescent. In the figure we can clearly see that on new moon day moon lies between the Earth and the Sun.
• It should be noted that time period between a full moon day and new moon day is slightly longer than 29 days. This time period in many calendar is known as month.
• Phases of the moon and its time period:

New Moon –3 ½ days –Waxing Crescent-- 7 days – First Quarter – 10.5 days – Waxing Gibbous – 14.5 days -- Full Moon

• On July 21, 1961 first person who was an American astronaut, Neil Armstrong, landed on the moon along with Edwin Aldrin (Buzz Aldrin) by Spaceflight Apollo 11.
• Sideral & Synoptic month: Moons makes a complete revolution around the Earth in 27 days, 7 hours, 43 minutes and 11.6 seconds or simply 27.3 days. This time period is called Sideral month. Whereas with reference to Sun, Moon’s period of revolution is 29 days, 12 hours, 44 minutes and 2.8 seconds or simply 29.53 days. This time period is known as synoptic month.