Question regarding date useage

Community General Discussion
1

I’ve been submitting photometry data for a number of years now and have always used the calendar date system. I am curious though regarding the Julian Day system: Is there a specific advantage to using Julian over the calendar date? And why does the Julian system even exist?

Any input is appreciated, I’m just curious…
Jamey Jenkins (JENJ)

2

Off the top of my head:

  1. It’s simple to calculate the difference between to dates, for example to calculate a variable star’s period.
  2. There is no confusion about which time zone the data was taken in. Everone (on Earth) uses the same JD
  3. For historical events, there is no confusion about Gregorian vs Julian calendars, and which was used for an observation.

Bonus Fact: there is a variation on JD called HJD (Heliocentric Julian Day). Don’t use this unless specifically requested, but it uses the time that an event happened relative to the Sun (technically, the barycenter of the solar system) instead of the Earth. That’s important when precision on the order of minutes is needed, so that the light-travel time across Earth’s orbit doesn’t affect the measurement.

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Hi Joseph…very good, I had considered the advantage of calculating a period, just simple subtraction. As always, now I know more than when I inquired. Thanks for the feedback!!!
Jamey (JENJ)

4

Strictly speaking — and one should always speak strictly, especially to animals and young children — HJD is now obsolete. It has been replaced by BJD — Barycentric Julian Date.

For real geeks, the time standard par excellence is Barycentric Dynamical Time, aka TDB, which includes relativistic corrections.

See Barycentric Julian Date - Wikipedia and Barycentric Dynamical Time - Wikipedia for more detail than you probably require.

Have fun,

Paul

5

We need to be careful here. It it not correct to say that HJD is obsolete, any more than it would be correct to say that JD is obsolete.

JD, HJD and BJD are simply different times. HJD differs from JD by a few minutes. BJD differs from HJD by about a minute or a bit more.

Each of these should (to be completely correct) be coupled with the time standard used. Usually JD is JD_UTC, and HJD is HJD_UTC. Typically, as Paul noted, BJD is BJD_TDB.

The time and time standard used for a particular set of observations will depend on the convention of the database in which the data is stored (for example, for the AAVSO International Database that is JD_UTC unless otherwise specified, and for the TESS database it is BJD_TDB), or the particular purpose of the observations (for example, most studies of changing periods in variable stars employing O-C diagrams will use HJD_UTC). For the most stringent requirements in professional astronomy BJD_TDB will be used.

4 Likes
6

Thank you Roy. My choice of words was poor and I apologise for that.

What I meant, of course, is what you stated about the delta-t between each system.

In a feeble defence of “obsolete”, I will suggest that the march of technology is starting to make the difference between HJD and BJD of interest to amateur astronomers. As you point out, the discrepancy can be around a minute. I suggest that when making observations of exoplanetary transits this is comparable to the precision now attainable and may well be of importance when looking for TTVs. Fortunately, the distance between the barycentre and the heliocentre is calculable to high precision over long periods and the light travel time between them likewise.

I suspect that both of us regard GR corrections to be unimportant on the timescales we use in practice.

Cheers,
Paul