# Locations of the terminator on Mars' surface on 1976-07-24T10:35:26Z?

How can I find points of latitude and longitude on Mars that fall on the terminator (day/night line) on a specific date, for example 1976-07-24T10:35:26Z as mentioned in this answer to Which Viking-1 orbiter photo is this? When was it taken and what part of Mars is shown?

I know that Horizons has rotational information of the planets and so I could pick points on it and observe the Sun and get its altitude above/below the horizon and through interpolation I could find the terminator, but perhaps there's an easier way.

Are there tables of epochs of say local noon at zero longitude? That could be interpolated smoothly and then just using the direction of Mars' axis and it's location relative to the Sun I could use geometry to get the terminator. Do such tables exist? Are there others ways to do this as well?

• I think you could use CSPICE to do this fairly easily. You'd still end up doing a binary search for every latitude, but it wouldn't require re-running the HORIZONS web interface every time. – user7073 Mar 26 '20 at 3:08
• @barrycarter That sounds like the most straightforward solution, elegance be damned. While there are certainly those who can run SPICE (C- or otherwise) easily, I'm not one of them. A binary search or even Newton's method would definitely get the job done, though when running a canned solver on the output of a black box one has to be careful: Why does scipy.optimize.minimize (default) report success without moving with Skyfield? – uhoh Mar 26 '20 at 3:18

[TL;DR] You can 1) use VTS Timeloop 2) use NASA's eyes on the solar system to visualize terminators on other planets at any time.

# 1) VTS Timeloop + Orekit

Instead of computing the terminator myself, I prefer to rely on visualization solutions such as the VTS Timeloop software from Spacebel/CNES which allows to visualize planets and space missions, offering a 2D view (usually for ground tracks) and a 3D view (using Celestia). It is not open-source but downloadable for free.

I modified one of my past projects to be able to visualize solar system planets from 1957 to 2057: https://github.com/GorgiAstro/planetary-timemachine. Instructions are available in README.rst. The VTS project is currently configured for Mars at 1976-07-24T10:35:26Z. For Mercury, Venus, the Earth and Mars, a WMS layer is configured for the 2D view, which means that higher resolution imagery is available when zooming.

More about SPICE and planetary ephemerides: The planetary ephemerides in VTS are defined via CIC files, which are basically CCSDS AEM and OEM files with a few differences. To visualize Mars back in 1976, I had to override the default VTS ephemerides though, which I did using the Orekit library instead of having to deal with SPICE kernels myself.

# 2) NASA's eyes on the solar system

I realized afterwards that this NASA software provides access to historical data of planets and interplanetary missions.

Just browse to 1976 and see Viking-1 orbiting Mars. The SPICE kernels are probably downloaded on the fly, so if Mars does not seem to rotate within a sol, it means that the SPICE kernel is still loading.

See some pictures below. The lighting effects on the terrain near the terminator look quite realistic. Although to answer the OP's question, the terminator location is not clearly marked here.

• Oh this is excellent! Yep those craters are near the terminator in the photo shown here and more importantly in this answer to Are there any photos of the Martian terminator from low Mars orbit? Thanks! It's great when a relatively new user digs in and posts an informative and well-sourced answer! – uhoh Apr 20 '20 at 22:53