Earth, Moon, Sun Earth, Moon, Sun SystemSystem

Earth, Moon, Sun SystemEarth, Moon, Sun System• As the moon revolves around the Earth, As the moon revolves around the Earth,

its apparent shape changes in a cyclic its apparent shape changes in a cyclic mannermanner

• The moon’s period of revolution is 27.3 The moon’s period of revolution is 27.3 days (The moon revolves approximately days (The moon revolves approximately 1313° per day in its orbit)° per day in its orbit)

• The moon also takes 27.3 days to The moon also takes 27.3 days to complete one rotation on its axis complete one rotation on its axis

• Therefore one lunar day is equal to one Therefore one lunar day is equal to one lunar yearlunar year

• On earth we can only see the On earth we can only see the portion of the moon that is portion of the moon that is illuminated by the sun. The illuminated by the sun. The

different shapes that we observe different shapes that we observe are called the PHASES OF THE are called the PHASES OF THE

MOONMOON

Phases of the MoonPhases of the Moon• New MoonNew Moon: When the moon lies : When the moon lies

between the earth and the sun, the dark between the earth and the sun, the dark side of the moon faces the earthside of the moon faces the earth

• Full MoonFull Moon: When the moon is on the : When the moon is on the opposite side of earth from the sun, the opposite side of earth from the sun, the whole lighted side is facing earthwhole lighted side is facing earth

• It takes 29.5 days for the moon to It takes 29.5 days for the moon to complete its phasescomplete its phases

Phases VideoAnimation 1Animation 2

Why does it take Why does it take longer to complete its longer to complete its phases than to make phases than to make

one revolution?one revolution?

• This occurs because as the This occurs because as the moon is orbiting the earth, moon is orbiting the earth, the earth is moving 1the earth is moving 1° per ° per day around the sun in its day around the sun in its own orbit. So the moon own orbit. So the moon

must move another 27.3° to must move another 27.3° to return to the same phase.return to the same phase.