# What is the unit of time “JD” in an astronomical context?

For my high school physics project I am required to research a planetary system. The orbital period of the planets are given in JD. What are these units? I've tried to research it, and I've found it could be Julian days (which is the same as a normal Earth day, right?) or Jupiter days, because every other datum is in terms of Jupiter's characteristics, e.g., mass of Jupiter and radius of Jupiter.

Here's the link a planet around my planetary system: GJ 667 C b. The notation is also used in the primary source.

• Your primary source gives orbital periods in days. – ProfRob Nov 13 '16 at 0:06
• Remember to check whether they are using the Julian or Gregorian calendars too (JD's can be defined in both) which can cause some confusion, especially in some software libraries that use both... – honeste_vivere Nov 17 '16 at 16:25

JD refers to Julian Date, which is a standard format for giving the times that astronomical events occur. If you are given a period in JD, then I guess that will refer to Julian days? Are you sure that you haven't got some thing of the form JD 2,451,545.1234 + 5.6789E

This is known as the ephemeris. The first number would be the Julian Date of a transit. The second is the orbital period in days.

And then I looked at your primary source and it gives the period in days (e.g. in tables 3-6). The JD refers only to the transit times. Where BJD is used, that refers to Barycentric JD which takes out the effect of the Earth's orbit around the solar system barycentre.

EDIT: i.e. The difference between BJD and JD is that the times are corrected for the light travel time difference between light reaching the Earth or the solar system barycentre. See Barycentric Julian Date. The maximum difference is 8.3 minutes either way.

• Can you expand on the difference between BJD and JD? Isn't the Julian date a fixed number of seconds after some given point in time? Or how is it affected by the Earth's orbit? – Emilio Pisanty Nov 17 '16 at 16:39
• @EmilioPisanty See edit. – ProfRob Nov 17 '16 at 17:26
• Ah, yes, of course. – Emilio Pisanty Nov 17 '16 at 17:41

Pending confirmation from a professional astronomer, this is almost certainly an abbreviation for Julian day, though it can also be a stand-in for Julian date. A Julian day is defined as exactly 24×60×60 seconds, which makes it a useful, non-variable yardstick for time (as opposed to e.g. solar or sidereal time, which changes with the Earth's orbit and rotation).