q & a

Here are some of the questions which visitors to Inconstant Moon have asked. They have been divided into six sections...

movement: the moon's orbit and phases
physical: form, characteristics and geology
phenomena: unusual lunar events
observation: what can be seen... and what can't
terminology: naming of the moon, phases and features
miscellaneous: anything not covered avove

If you cannot find the answer to your question, go ahead and ask it!

roche limit

q: What is Roche's Limit in miles? Will there be any effect on earth when the moon reaches maximum/minimum distances? (Dan Fortier, Tustin, California)
a: The Roche Limit, inside which an orbiting body will be broken up by gravitational forces, is 2.44 times the radius of the primary body measured from its centre. So for the Moon orbiting the Earth, the limit is 5700 miles (9200 km) above the Earth's surface. Artificial satellites have non-gravitational cohesion and so can travel within the limit. The Moon will never enter inside the limit as tidal forces are making it spiral slowly away from the Earth rather than towards it.

craters on earth

q: Considering the short distance between the earth and the moon when considering the distance between the sun and other plants in our system, why does the moon have so many craters? One would think that the Earth, as large as it is in comparison to the moon, would take more hits from objects in space, particularly considering our gravitational pull in comparison to the Moon. (Ron Waits, Wichita, KS)
a: In fact the Earth has indeed attracted more asteroids, meteoroids and comets than the Moon. The difference is that, while the Moon is airless, the Earth has a thick atmosphere which vaporises smaller bodies before they impact, and weathers away the craters left by the larger ones which get through. Nevertheless, geologists have discovered the remains of some 150 craters on the Earth's surface, some of them extremely large.

gravity & shape

q: Why do all of the planets and their moons appear round or spherical? What is the role of gravity? Does this also explain why naturally occurring objects on Earth, such as tree stumps, branches, nuts, berries, etc. are round? (Mary, Carpentersville, IL)
a: Gravity is a force which is very weak, but has an enormous range. The effect of this is that while small objects have an immeasurably small gravitational force, on an astronomical scale gravity is the dominant force. With progressively larger objects, gravity has a cumulative effect, and from the size of the largest asteroids upwards has the power to pull the object into a spherical shape. The round and spherical shapes exhibited by objects on a more earthbound scale are due to other processes.

rotation and revolution

q: The moon always has one side facing us. It revolves (around the Earth) and rotates (on its axis) in 29.5 days. Great. But why? What causes the periods to be the same? Is one side of the moon denser thus causing the heavy side to face the earth? This is strange. It can't be coincidence, or can it? The odds against coincidence are high. (Gary, Boca Raton)
a: It's definitely not a coincidence. Rotation and revolution period identical to any number of decimal places - that would take a lot of believing! In fact captured rotation, as it's called, happens to all orbiting bodies eventually, though the time taken depends on the masses of the bodies involved. You don't need to have greater density on one side of the satellite either. The mutual gravitational pull slightly distorts both the planet and its moon. As a gravitationally distorted body rotates and flexes (imagine rolling a cigar), the internal friction creates heat (eventually radiated into space), which robs it of rotational energy (the principle of conservation of energy). Over millions of years the rotation is slowed until it is locked facing its partner, which stops the flexing, heating and slowing.

Most of the moons in the solar system already exhibit captured rotation, so that they always show the same face to their primary planet. The ones that don't are generally captured asteroids that haven't had as much time to adjust. The planet Pluto also has captured rotation: it and its moon Charon are both locked facing each other as they revolve.

In about 9 billion years the Earth too will have had its rotation slowed so that the moon will appear fixed in the Earth's sky.

increased gravity

q: Suppose the gravity of the Earth increased. Meaning that, somehow, the Earth's mass increased or some other strange event occurred. Would the Moon be pulled closer to the earth, or would its orbital speed change? (Gale Griffin, Salt lake City)
a: As a hypothetical exercise in celestial mechanics - the Moon would divert towards the Earth onto a more elliptical orbit. It would accelerate as it approached perigee (the closest point). Assuming it did not actually collide with the Earth, it would then decelerate again as it swung back out towards apogee (the furthest point) which would be fairly close to the starting point of its new orbit. When it completed a full revolution, its velocity would be almost exactly the same as when it started.

asteroid impact on the moon

q: Everyone worries about what would happen if a large asteroid hits the earth. But what would happen if a large asteroid hits the moon? How would that affect the earth? (Janet Andretta, Palmyra, NJ)
a: Assuming it struck on the Earth-facing side of the Moon, it would be spectacular but essentially harmless. The asteroid which killed off the dinosaurs is now widely believed to have impacted at Chicxulub, on the Yucatan Peninsula, in the Gulf of Mexico. It left a crater120 miles (180 km) wide and 1 mile (1600 m) deep, similar in size to Maginus or Petavius on the Moon. You can gain an idea of how the impact might look from the account of the 11th century monk Gervaise of Canterbury, which is thought to describe the formation of a small lunar crater, and includes effects such as the creation of a temporary lunar atmosphere of dust, gas and vaporised rock.
It is probable that some of the ejecta would eventually reach the Earth, to fall as meteorites. Many of the meteorites found in places such as Antarctica originated from the Moon and even Mars.

orbital stability

q: Is it possible for the Moon to crash into the Earth?
a: Happily, the laws of Newtonian mechanics mean it will never happen. The faster a satellite travels, the greater its orbital distance, and vice versa. The Moon - with a mass of 7.353×1022 kg and a velocity of 1022 metres per second - has enormous momentum and would take a great deal of slowing down before it impacted the Earth... the equivalent of literally millions of large asteroid impacts, and all on the same side of the Moon!
In fact, because of an interesting effect of Earth's tides, the Moon is actually slowly spiralling away from the Earth.