Eccentricity Of The Moon's Orbit

This video was made with the Celestia astronomy program. The time of the view is November 2009. With a ruler the approximate eccentricity of the moon's orbit can be measured.

The ecliptic circle is the circle projected onto the distant stars by a line from Earth through the sun as Earth completes its orbit around the sun; the moon orbits Earth approximately in the plane of the ecliptic circle. The view direction of this video is from the north pole of the ecliptic circle, which can be seen from the fact that the moon appears to orbit approximately in a circle, in an anti-clockwise direction.

Celestia can be set up to duplicate this view. But how can we locate the north pole of the ecliptic? The j, k and l keys change the time of view with Celestia and the ';' key places a projection of lines of declination and right ascension on the background. That projection is derived from latitude and longitude on Earth. The view direction can be changed using the mouse with the right button pressed; holding both buttons and turning the mouse rotates the field of view. Numpad keys can be used also. The Home and End keys zoom towards and away from the selected object, in this case, Earth.

We know that the sun crosses the celestial equator - the projection of Earth's equator on the fixed stars - at the beginning of summer. Although the ecliptic circle doesn't appear on the version of Celestia used here, the point where the ecliptic and the celestial equator cross is the 0 point for right ascension, which is easy to locate. When the right mouse button is held down and the mouse is moved to the right, the view changes to the west. When the mouse is dragged down the view changes to the north, which can be seen clearly if Earth is kept in view while executing these changes. When such changes are made, Earth isn't moving because of it, it is only the view that changes. Movements of Earth and other objects are caused by changing the time of viewing.

Changing the view so that Earth appears at the 0 hour point gives the view of where the sun crosses the celestial equator moving northward and it would be beyond Earth as seen from here. Moving the view so that we are looking at the 6h line and then moving again so that we see a point 23.5 degrees north of there reaches the point where the sun is at its maximum distance from the celestial equator. This point is on the ecliptic circle and the north pole of the ecliptic circle is 90 degrees from there so we now change the view by 90 degrees to bring the south pole of the ecliptic circle and the Large Magellanic Cloud into view.

The moon's orbit is at an angle of about 5 degrees to the plane of the ecliptic and so it is better to keep Earth quite distant in the view for reasonably accurate measurements of the ellipticity. If anyone has the coordinates of the poles of the moon's orbit, please put them in the comments for this video.

Category:

Science & Technology

Tags:

• moon
• orbit
• ellipse
• elliptical
• celestial
• equator
• declination
• right
• ascension
• 23.5
• degrees
• ecliptic
• angle
• months
• lunar
• sun
• solar
• equinox
• solstice
• summer
• vernal
• pole
• hour

[eRazeTheRoof]

very informative. I now understand what a small pixel looks like travelling in a circle.

[andrewburbidge]

@eRazeTheRoof That was the moon. If you measure on the screen you can find the eccentricity of it's orbit - about 0.55.

[Mihorin51]

Thank you so much for that wonderful video. I was able to see the moon was very far from the earth, as I thought it was more near the earth.