The Moon and its Motions

The Earth’s Moon

Satellite: a body that orbits a larger bodySputnik I: the first artificial satellite (1957)1958: US launches 1st satellite, Explorer IMoon: a natural satellite of any planet

The Earth’s Moon

Moons gravity is 1/6 of that here on earth1969 – 1972 six Apollo missions to the moon - found moving to be quite differentGravity has never been strong enough to hold gases, so no atmosphereTemperature variation 134ºC to -170ºCVery slow rotation of 29.5 days (about 4 earth weeks)

The Lunar SurfaceLight and dark areasLight areas are rough highlands composed of light colored rockDark areas are called maria (Latin for sea) and are composed of dark solidified lavaRilles: long deep channels running through the maria

Rilles on the surface of the Moon

The Moons Craters

Craters: bowl shape depressionMost formed 4 billion years ago during the great bombardmentRays: displaced material from impact extending radially outward from the impact siteSome are as large as 250 kmLargest are named for famous scholars and scientists

Copernicus crater

Lunar Rocks

Surface covered by a thin layer of dust caused by meteorite impact – material in this layer called regolithDepth of this regolith varies from 1m to 6mSame elements as rocks on earth but in differing amounts

Lunar Rocks

Igneous rocks near surface are mostly oxygen and siliconAnorthosites: rocks from the highlands rich in calcium and aluminum – course grained and light-coloredRocks from the maria are dark-colored fine-grained basalts with titanium, magnesium, and iron

Lunar Rocks

Breccia: fragments of other rocks that have melted togetherLack elements with low melting points like sodium – burned off when the moon was moltenDo not contain water

Moon’s InteriorInformation comes from seismographsCrust on side facing us is 60 km thickCrust on opposite side is 100 km thick and very mountainousBeneath the crust is the mantle which is about 1000 km thick – lower portion may still be molten Core may have a small iron core with a radius of less than 700 kmNo overall magnetic field

Development of the Moon

Giant-impact hypothesis – Mars size body strikes the earth and ejected fragments eventually formed the moon2nd stage – covered by an ocean of molten rock – over time the minerals separated with those more dense materials migrating towards the core

The Moon’s Development

3rd stage – outer surface cools forming the crust – meteorite bombardmentImpacts decrease with timeGeologic activity stopped with the coolingMoon similar today as it was 3 billion years ago

Movements of the Moon

Lunar Movement

Orbit of the moon about the earth forms an ellipseApogee: when the moon is furthest from the earthPerigee: when the moon is closest to the earthAverage distance is 384,000 km (238,000 miles)

Lunar Movement

Moon appears to rise and set each nightChange of 50 minutes each dayMoon also spins on its own axisOrbiting around the earth every 27.3 days similar to its rotationWe see the same side of the moon all the timeMoon rocks on its axis so we can see about 59% of the surface

The Lunar Cycle

The Moon’s Phases

New Moon – moon is between the sun and the earth – no lighted area of the moonWaxing moon – when the amount of light on the surface we see is increasing – right side called a Waxing Crescent MoonFirst Quarter – half the lighted side is facing the earth – right half litWaxing-gibbous – more than half is lit

Phases of the Moon

Full moon – completely litWaning – light is beginning to decreaseWaning-gibbous moonLast QuarterWaning crescentEarthshine: sunlight that is reflected off the earthNew Moon to New Moon is 29.5 days

The Calendar

Day: time required for the earth to make one rotation on its axisMonth: time required for the moon to go through one cycle of phases as it orbits the earth (29.5 days)Solar Year: time required for the earth to make one orbit around the sun (365.24 days)

The Problems

365 days is too short for a year and 366 days is too long29 days is to short for a month and 30 days is too longMoon makes between 12 and 13 orbits around the earth in the time it take for the earth to orbit around the sunThe calendar must account for these differences

The Julian Calendar738 B.C. first calendar304 days divided into 10 months3 months behind the seasonsCaesar’s astronomers revised the calendarYear was 12 months – 11 months had 30 or 31

days and February had 29 days – later one day was moved from February to AugustLeap Year compensates for extra day needed every four years11 minutes longer than the actual solar year

The Gregorian CalendarBy 1580 the calendar was about 10 days ahead of the season – Pope Gregory XIII dropped 10 days from the month of October to compensateCalendar revised and currently in use in most of the world3 fewer days every 400 yearsYears ending in 00 and not divisible by 400 are not leap years (1700, 1800, 1900)Only 26 seconds longer than the solar year – only off by one day in 3000 years

Current Calendar Reform

Thirteen-month calendar – each month to contain 4 weeks – new month called Sol between June and July – one day called year day would fall at the end of each year – every 4th year leap day would be added to Sol.World Calendar – 12 months of 30 or 31 days – world day at the end of each year – leap day every 4 years at the end of June

Tides

Tides

The regular change in the height of the oceansCaused mainly by the moon and its gravitational pullThis pull is strongest on the side facing the moon and weaker on the other sideThe difference between the strong force on one side and the weak force on the other is called a differential gravitational force

TidesThe differential gravitational force draws water in the oceans into a tidal bulgeThere are two identical tidal bulges on either side of the EarthThe Earth’s rotation carries us into these bulgesAs we enter the bulge the water level rises and as we leave the water level fallsThis creates two high tides and two low tides per day

What happens when these tides encounter the land?

Water tends to pile up along the coastlineThe tidal bulge is typically about 2 metersThis can turn into 10 meters in a narrow bayThis may also cause water to rush up a coastal river in what is called a tidal bore

Lunar Motion and Tides

Motion of the moon in its orbit causes the tidal bulge to shift slightly from day to day.High tides come about 50 minutes later each day (remember this is the same delay as moonrise)

Solar Tides

The Sun also creates tides on EarthThe Sun’s tidal force is about 1/3 that of the moons because it is so much further awaySpring tides: abnormally large tides that occur during a new or full moonDuring spring tides the solar and lunar forces are working together

Solar Tides

Neap tides: tides that aren’t as extreme as even normal high and low tidesHappen during the 1st and 3rd quarter moonSun and moon are at right angles to each other and therefore lessen the tidal effect

Spring Tide

Neap Tide

Tidal BrakingTides create forces on both the Earth and Moon that slow their rotationWe call this slowing of the rotation tidal brakingAs Earth’s rotation slows, the moon accelerates in its orbit causing it to move further from Earth (about 3 cm per year)This means that the Moon was once much closer to Earth and that Earth spun much faster (as fast as a day being 5 hours long several billion years ago)Earth has now slowed to its present 24 hour dayTidal braking lengthens the day by 0.002 second every century

Tidal BrakingThis works on the Moon as well even though there is no water.Earth’s gravity creates tides on the Moon which are distortions in its crust that slow its rotation as wellThis is what locks the moon in the synchronous orbit and why we see the same face all the timeEventually the Earth will be in synchronous orbit too and we will only be able to see the moon from one side of the planet

Tidal Braking

The Moon’s gravitational pull on the Earth may also stabilize our climate by keeping the tilt of our axis fairly constantWithout the pull of Moon’s gravity our axis would change erratically and with it we would have large changes in the severity of the seasons

Moon LoreThe full moon triggers antisocial behavior (lunatics)Once in a Blue Moon – two full moons in a month – full moon on the first of the month – chances of this are 1 in 30 so this will happen about every 2.5 yearsMore to the blue moon – has to do with particles in the atmosphere – normally blue colors are filtered out – but if the particles are just right the red is filtered out and the blue passes through

Moon Lore

Harvest moon – the full moon nearest the autumn equinoxHunter’s moon – the full moon in October

Blue Moon

Harvest Moon