The RASC Calgary Centre - Barycenter Animation© 2006 Larry McNish, All Rights Reserved.Page last updated January 28, 2006 - Scroll down to read the explanation. The Earth-Moon Barycenter Animation (to scale)
This animation is a representation of the Earth-Moon system over a Lunar Month and is to scale. This page provides additional information regarding the question on its parent page - How Fast Are We Moving - Right Now? If you have not read that page I suggest you look at it now, then come back to this page. The legend at the top right of the animation explains what all the lines and circles are. Watch carefully as the Earth (the small blue circle in the centre) wobbles around the Earth-Moon barycenter (the tiny red dot exactly at the centre) each day over a period of one Full Moon to the next Full Moon. Viewpoint: You are traveling along with the Earth in its orbit, viewing from a fixed position high above the Earth's Northern hemisphere and perpendicular to the plane of the Earth-Moon orbit. The Earth proceeds counter-clockwise around the Sun (a direction up and to the left in this animation). From this vantage point you can see the relative positions of the Earth, the barycenter, and The Moon. It is not possible to show the Earth moving along its orbit line at this resolution, so the "observer" has to move with it. (The Earth moves about 202 times its own diameter in one day!) Scale: This animation was generated by a program using the correct object sizes and distances, so things should be "to scale" within a pixel on your monitor. The diagram would have to be considerably larger to show the Moon as more than the "dot" it is here. Note that the Moon's orbit appears to be a circle instead of a classical ellipse, simply because at this scale, the "flattening" of the ellipse due to an eccentricity of 0.0554 is less than one pixel. The barycenter's orbit around the Sun is so large (390 times larger than the Moon's orbit), that at this scale you cannot see a curvature in that line. Notes: Since this animation represents a Lunar month, the Earth-Moon system travels roughly 1/12th of the way around the Sun (nearly 30 degrees) during this time. This can be seen as the angle of the barycenter's orbit (the dashed line) changes over the time period. The Sun Angle - the angle from the barycenter to the centre of the Sun (shown as a yellow line) changes accordingly - always being at right angles to the orbital path. The Moon's orbit moves along with the Earth, but from this vantage point you can see the Moon's daily positions relative to the barycenter which is fixed in position at the centre of this animation. The animation starts at a Full Moon (on the far right side) and the Moon completes one 360 degree orbit in about 27.32 days (a Sidereal month). However, during this time, the Earth has progressed considerably along its curved orbit causing the Sun Angle to be about 30 degrees to the lower left of the Earth. Therefore, to return to opposition and again be lined up with the Earth and Sun for a second Full Moon, the Moon has to take an extra 2.2 days to get to the new position, or 29.530589 days (on average) overall. The planet Earth animation on the lower left shows the view from the Sun and how the Earth wobbles back and forth over the month relative to the barycenter which is represented by the red dot. The barycenter, of course, is located within the Earth, about 3/4 of the way from the centre to the surface. The animation on the lower right shows how the Moon looks from the Earth each day over the cycle from one Full Moon to the next. Accuracy: 1. This diagram was produced using the annual average values for positions and times. It does not represent any specific month. 2. The Moon's orbit is inclined 5.16 degrees with respect to the Earth's equator, which itself is inclined 23.4 degrees with respect to the Earth's orbital plane, but since we are viewing perpendicular to the plane of the Earth-Moon orbit it does not show any "foreshortening". | ||||
