Moon: Its General Properties and Motion

Moon

- It is the Earth’s only natural satellite.- It is the second densest satellite among

those densities are known.- It is the second brightest regularly visible

celestial object in the Earth’s sky.- It was formed 4.5 billion years ago from the

debris left over from an indirect collision between the Earth and an astronomical body the size of Mars.

General properties of the Moon

 Size

The diameter of the moon is 3479 km (2162 mi). This about ¼ of the Earth’s diameter.

Mass

The mass of the moon is 7.35x 10²² kg, which is about ⅛ of the Earth’s mass.

Density

The moon’s density is 3340 kg/m³ or 3.3x10⁴ g/cm³.  

Reflection

The moon’s average visual albedo is 12%, the brightness of the moon changes dramatically as its phases changes. During first and third quarter, the visible moon is 50% illuminated by the sun, but its brightness is only about 8% of the full moon.

Distance

Its current distance is about 30 times the diameter of the Earth, causing it to have an apparent size in the sky almost the same as that of the sun. This allows the moon to cover the sun nearly precisely in total solar eclipse.

Age

The moon is thought to have formed nearly 4.5 billion years ago from the debris left over after the collision between Earth and the Mars sized body.

Temperature

The average temperature on the surface of the moon during the day is 107°C. That is hot enough to boil water on Earth. During the night, the temperature drops to ¯153°C.

Gravity

Because of its smaller size and mass, the gravity of the moon is about the gravity of the ⅙Earth, that means that a person who weighs 180 lbs on Earth would only weigh 30 lbs, if measured on the moon.

Moon Causes Tides

The force of gravity from the moon affects the Earth. Its gravity reaches the Earth and pulls the ocean towards the moon, causing tides. The highest tides will occur when the moon and sun are aligned and that is when there is New Moon or a Full moon.

Motion of the Earth’s Moon

- the motion of the moon is always synchronous to the Earth.

- The moon moves 13.2° per day relative to the stars which is just over half a degree per hour and approximately equal to its apparent size.

In order to be seen again in the same phase, i.e., in order to return in the same position with respect to the stars, the moon has to turn a little bit more than just one lap around the Earth, while 29 days, 12 hours, and 44 minutes need to go by between two New Moons. This period of the time is called “Synocdic Month” or “Lunation.

Motion of the Moon: Rotation of the Moon on its Axis: