# Does the arrow in this STEREO trajectory animation point heliocentric prograde, or towards the Sun?

This answer links to this NASA page. The first video shows the two STEREO spacecraft use two and three passes by the Moon to leave Earth orbit and enter into an orbit around the sun.

Below is a GIF hand-made from the frames of that .mov file, but you can download the tiny .mov files from the website.

The text mentions:

When the two STEREO spacecraft are launched, they are first placed in a highly eccentric orbit around Earth. In this Quicktime movie, the green dot represents Earth, the white dot stands for the Moon, and the letters "A" (red) and "B" (blue) stand for the STEREO "Ahead" and "Behind" spacecraft respectively. The yellow arrow represents the direction of the Sun, which changes as Earth moves along its orbit.

To me it looks like the arrow points in the prograde direction, the direction that the Earth is moving around the Sun, not towards the Sun.

Am I wrong?

• I suspect you're right. Why not email Therese for clarification?
– user20636
Apr 16, 2019 at 9:46
• @JCRM That could be asked of any of the 21,376 individual users of this site, of which you are one!
– uhoh
Apr 16, 2019 at 9:48
• The term you're looking for is "apex", which refers to the direction of earth's motion in solar orbit. Apr 16, 2019 at 14:18
• @Tristan that's interesting, I've never heard it used before as a direction. Maybe it can be added here as well? I don't know how to use it correctly in a sentence; for example "Does this arrow point heliocentric apex? Feel free to edit my question if that's faster.
– uhoh
Apr 16, 2019 at 14:24

Am I wrong?

Yes, I am wrong!

The way the SOHO A and B pair move roughly symmetrically ahead and behind the Earth is by being in orbits similar to that of Earth but slightly closer and farther from the Sun. Thus when seen the GIF in the question which is in a frame that moving along with the Earth (but not rotating) at ~30 km/s it looks like A and B are leave cis-lunar space directly towards and away from the Sun.

But looking in a rotating frame with the Earth at the origin (Python plots below), it's clear that as soon as they move a bit in those directions, orbital mechanics takes over and they start along their "Ahead" and "Behind" orbits.

Answer: The arrow points towards the Sun just like the animation says. The folks at NASA who successfully launched the probes and threaded those precision lunar flyby's could in fact point an arrow in the right direction as well!

The Python plots below of data from JPL's Horizons web service. Units are AU, Sun is up.

The GIF is a string of images from https://stereo-ssc.nascom.nasa.gov/cgi-bin/make_where_gif