Add support for Barycentric Julian Date

[vterron]

This is the BJD_TDB FITS keyword. It's a non-standard keyword but commonly used for exoplanets.

More information on:

• https://en.wikipedia.org/wiki/Barycentric_Julian_Date
• http://astroutils.astronomy.ohio-state.edu/time/

[vterron]

We'll need the function bjd2utc(), as the input FITS images will use Barycentric Julian Date while LEMON works internally with seconds after the Unix epoch.

[vterron]

From Achieving Better Than 1 Minute Accuracy in the Heliocentric and Barycentric Julian Dates:

The biggest source of confusion comes from the fact that time standards and reference frames are independent from one another, even though there are many overlapping concepts between the two. We will use the following terminology: “reference frame” will refer to the geometric location from which one could measure time – different reference frames differ by the light-travel time between them; “time standard” will refer to the way a particular clock ticks and its arbitrary zero point, as defined by international standards; and “time stamp” is the combination of the two, and determines the timing accuracy of the event.

This is a nice article.

[stale[bot]]

Thanks for contributing to this issue. As it has been 180 days since the last activity, we'll be automatically closing the issue soon. This is often because the request was already solved in some way and it just wasn't updated or it's just no longer relevant. If that's not the case, please respond here within the next 30 days to keep it open. You can read more here: https://github.com/probot/stale#is-closing-stale-issues-really-a-good-idea.