- 1,384

- 2

(1) If you want a slightly bigger challenge on the first question, rather than just average speed, you can calculate the moon’s speed at a particular point on its orbit, using the vis viva equation.

Taking the moon’s current distance from http://www.wolframalpha.com/input/pod.jsp?id=MSP84219d1c35216ihb377000043gf3i5i495496cb&s=54 [Broken], I googled

sqrt(G((mass of earth)+(mass of moon))(1/m)(2/364717584-1/384472282)) in km/h

to get 3 882.96569 km/h. A bit faster than your average value, which seems reasonable, since the moon is currently closer than its average distance.

(2) There are some figures relating the movement of sun and earth to the cosmic microwave background (CMB) here:

Suppose we have a figure for the moon’s current speed relative to anything. What will it be without gravity? Well, if the only thing we change is to remove gravity, the moon’s speed will be exactly the same until its motion is effected by some other force such as the pressure of the solar wind. If we remove all external forces, its velocity shouldn’t change at all.

Taking the moon’s current distance from http://www.wolframalpha.com/input/pod.jsp?id=MSP84219d1c35216ihb377000043gf3i5i495496cb&s=54 [Broken], I googled

sqrt(G((mass of earth)+(mass of moon))(1/m)(2/364717584-1/384472282)) in km/h

to get 3 882.96569 km/h. A bit faster than your average value, which seems reasonable, since the moon is currently closer than its average distance.

(2) There are some figures relating the movement of sun and earth to the cosmic microwave background (CMB) here:

So the speed of the moon about the earth is a couple of orders of magnitude smaller than the seasonal difference due to differences in the earth’s linear direction of motion, and also somewhat smaller than the uncertainty given here for the sun’s speed relative to a coordinate system where the CMB is isotropic (the same in all directions).The speed of the Sun relative to the CMB is now 1,328,000 km/h (plus or minus 9000 km/h) in the direction of the constellation of the Cup [Crater] (just south of the Lion [Leo]). Contributions 2 - 4 make your speed relative to the CMB up to about 105,000 km/h greater (greatest around 15 December, if my calculations are correct) or smaller (least around 12 June) than the speed of the Sun “relative to the CMB”, mostly depending on the season.

Suppose we have a figure for the moon’s current speed relative to anything. What will it be without gravity? Well, if the only thing we change is to remove gravity, the moon’s speed will be exactly the same until its motion is effected by some other force such as the pressure of the solar wind. If we remove all external forces, its velocity shouldn’t change at all.

Last edited by a moderator: