The Moon

Notes on Chapter 6

The Moon’s diameter is about ¼ the size of Earth’s.

The Moon orbits Earth at an average distance of about 380,000 km, and has an orbital period of 27.3 days*.

The Moon has a diameter of 3,476 km, and has 1/6 of Earth’s gravity.

*It takes 29.5 days for the Moon to go through a full cycle of phases. This is similar to why sidereal days are different than solar days.

The Moon’s surface is divided into bright highlands and dark maria (Latin for seas). It is covered with craters, rays and rilles.

The highlands are rich in calcium and aluminum silicates – lighter materials than those found in maria.

Highlands are also covered in craters, which are caused by impacts on the surface.

The craters are very old – there is no weather or plate tectonics on the Moon to wear them away.

Rays are long, bright streaks of pulverized rocks that radiate outward from craters.

They form during the impact process.

When an impacting body hits a solid surface, it disintegrates into vaporized rock and fragments.

The crater left is circular; the size depends on the mass and velocity of the object.

The vaporized rock expands, pushing the surrounding rock outwards to form a raised rim.

Rock particles are thrown out from the crater to form rays.

Sometimes the rock below the impact is compressed enough that it bounces back to form a central peak.

Rilles are lunar canyons. They may have been caused by ancient lava flows or cracking of the crust.

Maria are dark areas that are made up of cooled lava. The rocks here are denser than in the highlands, and contain iron, magnesium, and titanium silicates.

Maria are much younger than the highlands – which means that they formed later.

While the Moon’s crust was fairly thin, it was hit by several very large objects (over 100km across).

They broke through the crust, making huge craters and pushing up mountain chains along their edges.

The craters then filled with lava, which cooled to form the maria.

By the time the maria formed, there were not many bodies left to hit the Moon.

As a result, the maria are fairly smooth.

Regolith – shattered rock (fragments and powder) from impacts. Up to several hundred meters thick.

Crust – silicate rock, like Earth’s crust. ≈100 km thick, but thinner (65 km) on the side facing Earth.

Mantle – solid rock (probably same type as Earth’s mantle). Too cold and rigid for convection. ≈1000 km thick

Core – much smaller and with far less Ni and Fe than Earth. (No magnetic field.)

The lunar sky is black because there is no atmosphere on the Moon.

As a result of this, daytime temps soar (over 100OC) and fall drastically (below -200OC) at night.

The Moon’s lack of atmosphere is due to…

…no volcanic activity* to provided gases.

…gravity that is too weak to hold onto any atm.

that forms.

*The interior is too cold and too solid for plate tectonics to occur.

Finding the distance to the Moon by triangulation and laser ranging.

The Moon is locked in a synchronous orbit by the Earth’s gravity. This means that the same side of the Moon always faces Earth.

The Moon’s orbit around the Earth is tilted about 5O with respect to the Earth’s orbit around the Sun.

Scientists believe that the Moon formed as a result of a collision between the Earth and a Mars-sized planetesimal about 4.5 billion years ago.

Initially, much of the debris rained down to form the cratered highlands.

Later on, a few large remnants hit, forming the maria.

After that, there wasn’t much debris left to form new craters.

An eclipse occurs when the Sun, Earth, and Moon all line up.

Lunar Eclipse – Moon goes through Earth’s shadow.

Solar Eclipse – Earth goes through Moon’s shadow.

Eclipses don’t happen every month because the Moon’s orbit is tilted.

Both solar and lunar eclipses occur. These seasons are about 20 days earlier each year.

Eclipse seasons usually happen twice per year, and about 6 months apart.

Solar Eclipses

Partial

Annular

Total

Oct. 23, 2005

March 29, 2006

Lunar Eclipses

The tides are caused by the differential gravitational force of the Moon on various parts of the Earth.

The Moon’s gravity is strongest here – it pulls the ocean away from the Earth.

This creates a tidal bulge.

The Moon’s gravity is weaker here – it pulls the Earth away from the ocean.

High TideHigh Tide

Low Tide

Low Tide

The Sun causes tides, as well, but it’s effect is only about half as strong as the Moon’s.

Spring Tides – larger than average bulge.

The Sun causes tides, as well, but it’s effect is only about half as strong as the Moon’s.

Neap Tides – smaller than average bulge.

Tidal BrakingEarth spins faster than the Moon orbits.

This means that the Earth turns beneath the tidal bulges.

This creates friction between the ocean and the ocean floor.

This drag slows the Earth’s spin, making the day longer.*

It also pulls the Moon, speeding up its orbit. This allows the Moon to pull away from Earth’s gravity.**

* By 0.0023 sec per century.

** By 3 cm (1”) per year.

Tidal Braking

Eventually, tidal braking will cause the Earth and Moon synchronous with each other. The same side of the Earth will always face the same side of the Moon.

Names Used for Full MoonsJanuary Old Moon July Thunder or Hay Moon

February Hunger Moon August Grain or Green Corn Moon

March Sap or Crow Moon September Harvest Moon

April Egg or Grass Moon October Hunter’s Moon

May Planting Moon November Frost or Beaver Moon

June Flower Moon December Long Night Moon

http://flushrush.com/full-moon-names/

Dust from volcanoes and forest fires can filter red light and cause the Moon to appear blue.

This happens very rarely, hence the expression “once in a blue moon.”

Also, a “blue moon” is the second full moon in the same month. This only happens when the full moon falls on the first of the month.