# Has the Earth's wobble around the Earth-Moon barycenter ever been observed by a spacecraft?

Pluto's motion around the Pluto-Charon barycenter has been imaged by the New Horizons spacecraft:

Has anything like this been imaged of Earth? Yes, the barycenter lies inside of Earth, but it is 3/4th of the distance from the center — more than enough for a noticeable wobble over the course of one orbit.

• Accurately determining the actual motion of the Earth is coordinated by the IERS. The distance to the Moon is found using lasers by the ILRS. For some details, please see this PDF. Jan 20, 2021 at 0:18
• For some values of imaged.
– Mast
Jan 20, 2021 at 18:14
• Out of curiosity, when you say “wobble” do you mean the Earth’s periodic nutation? Jan 22, 2021 at 7:13

Has the Earth's wobble around the Earth-Moon barycenter ever been observed by a spacecraft?

Absolutely yes if you can count the wobbling motion of the spacecraft that tracks Earth's wobble about the Earth-Moon barycenter.

DSCOVR sits in a heliocentric orbit which is in a 1:1 resonance with Earth, otherwise known as a "Lissajous orbit associated with Sun-Earth L1".

Its EPIC camera is pointed carefully at Earth all the time in order to keep a 0.45° to 0.53° Earth nicely framed in its only slightly larger 0.61° field of view.

If the Earth-Moon average distance is 385,000 km and they have a mass ratio of 81:1, Earth's wobble should be roughly 4,800 km or about 38% of its diameter. If DSCOVR didn't track this wobble the Earth would appear to move +/- 0.18° in EPIC's field of view such that parts of Earth would disappear outside the frame around the times of first and third quarter of the Moon as seen from Earth.

One can download roughly daily data dumps from EPIC which contain data fields for the pointing vectors of the camera. One can subtract a smoothly varying rotation of $$2 \pi$$ per year and see both Earth's eccentricity and a monthly wobble due to center-of-mass motion of the Earth. I've looked at EPIC metadata in the past, and while I haven't done this specific analysis this can't not be true!

above: Image from NASA/NOAA from Bad Astronomy

• Good answer, so from the earth-sun L1 we see the barycentric wobble as a perspective varying back and forth, rather than around viewed from the top. here is an illustration of DSCOVR orbit. youtu.be/0WN7lS9bpB4?t=87 The DSCOVR orbit will also move at synchronously with the wobble of the earth, and if it's as much as 4000km then it wouldn't actually see the barycentric wobble because it would be varying on the same barycenter. Jan 20, 2021 at 2:29
• @aliential the Moon has an inclination of about 5 degrees to the ecliptic, usually passing well above or below the Earth out of sight (perhaps +/- 30,000 km). The lunar "photobomb" in the GIF is actually an exception. Also DSCOVR's orbit has an even larger out-of-plane excursion, so if DSCOVR could image the moon it will see a squiggly Lissajous-like pattern, rather than a nice elliptical wiggle.
– uhoh
Jan 20, 2021 at 3:26
• @aliential I'm not sure how large the "synchronously with the wobble of the earth" effect will be, the orbital plots in the video might be geocentric rather than Earth-Moon barycentric, the knees in the orbit may be an artifact of the choice of frame for that plot.
– uhoh
Jan 20, 2021 at 3:27
• There are nasa videos showing 12 months of timelspse from dscovr, including a solar eclipse... the wobble is perhaps in line with the solar system, the video states that the sun rises and sets 13 times per day, so perhsps the l1 orbit is fast. Perhaps the ~13 sunsets per day and photos everyb ~2 hours are sunchronised. Jan 20, 2021 at 6:40
• If DSCOVR is 80 million miles from the sun and 1 million miles from us, then it is balancing 50/50 on sun and earth gravity, so perhaps it would follow the earth's wobble of 4600km by 2300km. here are some NASA videos. youtube.com/results?search_query=dscovr+timelapse Jan 20, 2021 at 7:45

I think that the answer is no. Here is why:

The image you gave was photographed when New Horizons was getting closer and closer from a large distance. (It wasn't stationary.) We can't easily get closer and closer to the Earth with some camera (expensive to sent something away and then closer), but we can get away from the Earth and photograph it. The problem is, that the orbital period of Charon is around 150 hours, but orbital period of the Moon is around 30 days. The camera would be too far from the Earth to picture that for one revolution or even the fraction.

• How is it important that the camera got closer? Jan 20, 2021 at 7:54
• It isn't actually that important, but it is very hard to stop in the middle of the solar system, right? Jan 20, 2021 at 8:38
• I haven't included the Lagrange points, but L1 is too close, the others are too far to detect it. @uhoh included the answer for the L1, but it isn't placed on top as his image. Jan 20, 2021 at 8:40
• @User123 I agree and I think that your answer is better than mine because it addresses the question directly. The view from the side will include a small amount of vertical motion because DSCOVR's motion takes up and down through the Earth-Moon orbital plane, but mostly it will be left-right oscillations. I can't think of any spacecraft that spent months well above or below the Earth-Moon orbital plane that was still close enough and well-equipped enough to make an image like the one in the question.
– uhoh
Jan 21, 2021 at 23:10