 | |||||||||||||||||||||||||||||||
Theory Observation
|
The Earth-Moon System
| ||||||||||||||||||||||||||||||
| Distance From Earth (in miles) | 239,000 |
| Perigee (in miles) | 226,000 |
| Apogee (in miles) | 252,000 |
| Orbital Inclination | in ° compared to the ecliptic: 5.1° in ° compared to the Earth's equatorial plane: 18.28°-28.58° (1) |
| Orbital Eccentricity | 0.055 |
| Axial Tilt (in °) | 6.7 |
| Mass (in tons21) | 0.081 |
| Diameter (in miles) | 2159 |
| Density (in lbs/ft3) | 209 |
| Mean Temperature (in F) | -4 |
| Magnetic Field | no |
figures courtesy NASA's National Space Science Data Center (NSSDC)
(1) back the range of the inclination relative to the terrestrial equator matches the fact that the line of nodes of Moon's orbit is regressing (moving clockwise) in 18.6 years
Values Explanations
- Apogee
- Point of the Moon's orbit which is farthest from the Earth (in miles)
- Axial Tilt
- Angle between the poles' axis of the Moon and a line perpendicular to the Moon's orbit's plane (or -this is the same, the angle between the equatorial plane of the Moon and the Moon's orbit's plane) (in °)
- Density
- Mean density (mass divided by volume) (in lbs/ft3)
- Distance from Earth
- Mean distance from the Earth (in miles)
- Diameter
- Diameter of the Moon at the equator (in miles)
- Magnetic Field
- Whether or not the planet has a measurable global magnetic field (Moon has no global magnetic field but localized magnetic fields)
- Mass
- Mass of the Moon (in tons21). Mass is not weight. Mass is the amount of matter in an object. Weight is mass on which gravity is acting. E.g. an object of similar mass (154 lb.) may have a weight of 154 lb. on Earth by on 26 lb only on Moon where gravity is 1/6th the one of Earth. Mass is used to characterize an object's inertia to acceleration, i.e. its resistance to be moved or have its direction changed
- Mean Temperature
- Mean temperature for the whole of the surface of the Moon (in F)
- Orbital Eccentricity
- Measurement of the circularity of the orbit (like planets, Moon has an elliptical orbit). A circular orbit would have a value 0
- Orbital Inclination
- Angle between the Moon's orbit's plane and the ecliptic (which is the plane of the Earth's orbit) (in ° compared to the ecliptic which is 0 and in ° relative to the terrestrial equator)
- Perigee
- Point of the Moon's orbit which is nearest to the Earth (in miles)
The Earth-Moon System
The Earth-Moon system appeared early in the history of Earth, as one of the theories about the Moon's origin is that a planetesimal the size of Mars came and hit the Earth about 4.5 billion, about 30 to 50 million years only after the Sun and the Earth were born. It ripped off the Earth's crust, sending a swarm of debris in orbit. Such debris eventually settled into a huge ring of debris, which lasted a year or so, as the ring further accreted back, forming the Moon. The Moon was quickly locked into its orbit around Earth in that way that its orbital period matched its rotational period. This occurred in a few tens of millions of years. Like today, Moon orbited at the same time that it rotated, hence presenting always the same face towards Earth. This was due to the large mass of Earth and the larger rotational period of it
On the other hand, at the same time, both Earth and Moon were much closer, and were each rotating more rapidly than today. Moon was certainly about between 12,000-29,000 miles (19,000-47,000 km) away from Earth only, yielding a much swifter orbit than today (about 4.19 hours), as Earth was rotating a few hours less than today, yielding a length of day at the early Earth shorter by the same amount. Dinosaurs, 70 million years ago were still having a day shorter than today by 2 hours
Along the eons, both bodies continued to interact. As Moon is orbiting Earth, it makes appear a tidal bulge at Earth -the tides as well as a real crustal bulge. As the Earth is rotating, this bulge in turn exerts a gravitational effect on Moon, accelerating it on its orbit as it is very slowly acting on the length of the day. Actually this translates into the Moon getting further from Earth, hence slowing its orbital period as the Earth's day is slowly getting longer. Moon is getting distant by 1.49" (3.8 cm) per year as Earth's day is slowing by 0.002 seconds per century. This will eventually lead to that Earth and Moon will get tidally-locked at 47 days. Moon will orbit in 47 days, as Earth's day will be of 47 days too! This should not take place before several tens of billions of years however. At the present Earth's rotation rate, it takes 3 million years to get the day 1-minute longer. At the end of the evolution, Moon will be 348,000 miles (560,000 km) away from the Earth, instead of 239,000 (385,000 km) today
 |
| the Moon's apparent diameter today and in 1 to 2 billion years from now (left); a total Sun eclipse today et a eclipse in 1 to 2 billion years (right) |
A by-side effect of this evolution is that solar eclipses, at one point, will cease to exist. This should occur definitively about 1 to 2 billion years from now. The process will occur gradually as total eclipses will slowly turn into hybrid, then annular only. At the end of the evolution, there will be annular eclipses only, with the eclipsed Sun visible by 4.8' each side of the occulting Moon, and partial eclipses. The decrease rate of total eclipse is rated this way: an incremental decrease of the apparent diameter of the Moon of 1.8" brings an average of one eclipse per century to cease being total to become hybrid.