How do scientists know exactly how far the Moon is away from the Earth?

Answer 1

They can measure by bouncing laser beams off of a mirror placed during the Apollo 11 mission specifically for this purpose. Read here for more info:
http://www.lpi.usra.edu/expmoon/Apollo11/A11_Experiments_LRRR.html

Answer 2

No one knows EXACTLY how far the Moon is away from the Earth. The distance is constantly changing. On average, the Moon is receding from the Earth because it loses energy creating tides on Earth.

The best way to accurately measure the distance at any particular moment is with a good telescope fitted with a laser, a configuration called a LIDAR which is analogous to RADAR except shorter wavelengths are used. To avoid the complication of the Moon not being a particularly good reflector (look at some moon dust brought back by the astronauts to see why), Apollo 11 crew left an optical retro-reflector on the Moon to aim lasers at. This is just three very flat mirrors arranged so they are all mutually perpendicular to each other, sometimes called a corner-cube reflector. It has the neat property that any beam of light, coming from any direction, that hits the retro-reflector will be reflected back in the SAME direction. The end result is a lot more light arrives back on Earth to be detected for range measurement.

To actually make the range measurement the laser is pulsed on for a short period of time, a few microseconds at most, and then the round-trip time of the reflected pulse is measured accurately. Multiply by the speed of light in vacuum and divide by two to get the distance, accurate to perhaps plus or minus a thousand feet or so without going to too much trouble.

Answer 3

Firstly, the orbital distance of the moon varies throughout the year by about 10%.The average distance is about 384,000 km.

How was it measured? There are several ways.

The Lunar expeditions of late in the 1970 left retroreflectors on the Moon, so one can send a laser beam to them and measure the round trip time. since light travels at about 300,000 km/s and it is possible to measure time to better than one billionth of a second called one nanosecond ("ns" for short). Light travels about 30cm or 1 foot in 1 ns; and because they measure the round trip, that one nanopsecond corresponds to a distance measurement of 15 cm (6 ").

Prior to that, the distance was measured using radars, using time of roundtrip measurements. Like light, the radar radiation travels at the speed of ligh, which allowed for deistance determination of about 15 cm.

How was the distance measured in the 17-th Century? Based on the Newtons Law of General Gravitation (see any grade 11 physics textbook) the distance was calculated from the known orbital operiod of the Moon

R^3 = T^2 x G x M / 4(pi)^2

R - distance from the centre of Earth to the centre of Moon
T - Orbital period
G - Universal Gravitational Constant
M - Mass of the Earth

And in the ancient time?
By comparing the apparent angle of the Moon from two different places on the Earth. That method was first usedfor measurement of the Earth's size (to within about 20% of the current value) by the Greek and Egyptian astronomers, and also, may have been known to the Chaldeans (Babylonians) about 2500 years ago.

Answer 4

There's two main ways....

1. Radar

2. Laser 'radar' . The astronauts who DID land on the moon left behind special reflectors to aid this task. from earth a pulse of laser light is sent to the moon, it's reflected back in exactly the same direction by these reflectors, and the time difference between transmission and reception enable the distance to be calculated to within two fifths of a gnat's tadger.

Answer 5

When the astronauts visited the moon, they left mirrors on the surface. Scientists can shoot a laser at the moon, bounce it off one of the mirrors, and measure how long it takes for the light to get there and back.

Answer 6

they send out a signal at a certain frequency, it bounces off the moon, then they measure how long it takes to get that signal back. Then scientists convert the time into distance. There ya go.

Answer 7

Bounce radiowaves off it ?

But it was worked out before radio waves were discovered i think.

However - I didn't know why the moon shines white... until my brother told me its because its lit by light of the sun.

How would you do measure the distance hmm..... maybe tides give a clue.

Update: This USA astronaut mirror thing I do believe - but only to measure very exact distance. They must have had a very close idea of the distance before setting off from earth. :p

Answer 8

In physics this semester we learned all about center of mass and orbital periods. Physicists can calculate the distance from here to the moon using all kinds of complicated physics formulas that deal with those.

Also, I'm sure they can use some type of laser to measure the correct distance.

Answer 9

If you have read answers by me previously you willo know that I currently live on the moon. I do come back to earth a few times each year so I keep the scientists updated regularly on the exact distance I travel each time. Armed with this depending on planetary cycles they can calculate the true distance. I hope this helps you.

Answer 10

One of the earlier way to do it was like using kepler law that state that the revolution time is the inverse square of the distance, so by knowing how long an object take to orbit another you can deduce the distance, others methods came after that where more precise, but usualy with newton's and kepler's law you can resolve all theses problems.

Answer 11

the last time the U.S.A. went to the moon they put mirrors on the moon they use a laser to calculate how fare the moon is away from the earth