# Which side of Europa is which?

I understand that Europa is tidally locked* to Jupiter, so there's a Jovianmost longitude on one side and an anti-Jovian longitude on the other.

What I can't figure out is which side is which. Looking at a map of Europa, is Dyfed Regio (for example) right near the Jovian longitude or the anti-Jovian longitude?

*I've read some articles that say Europa might precess more than we previously expected. For this question, that doesn't matter. I'm just asking about its current orientation.

The standard for any tidally locked body, of which Europa is a member, is to have the 0 longitude be the point at the center of the planet-facing side. That being the case, the middle of the map should be the portion facing Jupiter, the edges the part that never faces Jupiter. See Wikipedia for the referenced quote below:

Tidally-locked bodies have a natural reference longitude passing through the point nearest to their parent body: 0° the center of the primary-facing hemisphere, 90° the center of the leading hemisphere, 180° the center of the anti-primary hemisphere, and 270° the center of the trailing hemisphere.

That being said, the source image for your map is found at the USGS website. The far right side is 0 degrees longitude, the far left 360, of the image shown below:

The darker region, which is the central piece for your image, is centered around 250 degrees longitude. In fact, looking at the database of features on Europa, we find out that Dyfed Regio is 10 lat, 250 longitude.

• So that means Dyfed Regio is in the middle of the anti-Jovian hemisphere? (I just want to make sure I'm understanding correctly.)
– Joe
Jul 14, 2016 at 1:28
• @Joe: Added a bunch more to my answer, it's much better now. Jul 14, 2016 at 1:39
• So I have heard that anyone can just call up the USGS and order one of their maps for free, can anyone verify? Because I would really like a copy of that map. Jul 14, 2016 at 4:18
• Never heard of that. You can buy 'em from their store for \$9, which isn't bad. Jul 25, 2016 at 4:50
• Does the dark spot align with the velocity vector? Apr 10, 2019 at 17:41

I find the accepted answer unclear, so I'll try:

The two maps in discussion use coordinate systems based on the convention that 0 longitude is the point directly facing Jupiter, but they have chosen to put that point on the far right-hand edge of the map.

Dyfed Regio is on Europa's Trailing and Anti-Jovian hemispheres (not the Sub-Jovian).

Note that the maps posted by @PearsonArtPhoto and that posted by @Joe have different labels (e.g. the USGS one doesn't have regia) but do use the same coordinate system. Here's a figure from https://doi.org/10.1016/j.icarus.2018.04.016 which also uses the same coordinate system and could be helpful: