# What does BJD-2450000 mean?

In different papers I've encountered time indications in BJD-2450000. I know that BJD stands for Barycentric Julian Date, but I don't understand the meaning of the suffix -2450000.

This is an example of what I'm talking about (table is taken from this paper). I'd like to know how I can convert this in JD or just "normal" BJD if that makes sense?

Julian dates offer one nice feature: All astronomical observations recorded by humankind have a positive timestamp. A key downside of Julian dates is that current timestamps on computers that use 64 bit IEEE Standard for Floating-Point Arithmetic (IEEE 754) have a resolution of 40 microseconds. Almost all computers use the IEEE 754 floating point standard. This lack of precision can be a problem with some modern astronomical observations.

One way around this limitation is to use a pair double precision numbers to represent time, the sum of which logically represent the Julian date. This is the approach used by the Standards of Fundamental Astronomy (SOFA). Another approach is to use an offset.

One example is Modified Julian Date, subtracts 2400000.5 from the Julian date. This slices off the leading 24 which is always present on any recent Julian date (JD 2400000.5 was midnight on 17 November 1858, and JD 2500000.0 will occur at noon on 31 August 2132). The additional half of a day in the MJD offset effectively sets the start of a day at midnight rather than at noon.

Subtracting 2450000.0 is equivalent the slicing off the leading 245 from recent Julian dates. This simple slicing technique is valid from noon 9 October 1995 (inclusive) to noon 24 February 2023 (exclusive). The precision using IEEE 754 doubles at the latter date will be two picoseconds, which is more than precise enough for timestamping any astronomical observation for the past two decades and the next decade.

That is a simple offset in order to work with smaller numbers. Add that number (2450000) again to each value in the day column and you have the unmodified value of BJD.

• To say it another way, the “-“ in the column header is a minus sign (not a dash), so what is tabulated there is BJD minus 2,450,000. – Eric Jensen Jul 17 '20 at 14:46

The advantages of using Julian dates (and what to do with the "- 2450000" part) have been described well in previous answers but if you're curious about the "barycentric" part here's an example.

Let's say I'm observing an eclipsing binary to measure the period which might be changing. Six months later I observe the system again. The measurements could be off by up to ~15 mins (the diameter of the Earth's orbit in light minutes) because I'm on the other side of the Earth's orbit now. Astronomers started using "heliocentric" Julian dates (HJD) to take care of the problem - as if the observatory was located in the center of the Sun - so the location of the Earth in its orbit was no longer a source of error.

HJDs weren't a perfect solution; the Sun doesn't sit in the middle of the solar system without moving. The Sun, planets, and all of the objects in our solar system are orbiting the center of mass of the solar system which is called the barycenter. Now Julian dates are corrected as if the observatories are located at the solar system barycenter instead. The difference between BJD and HJD is much smaller, around ± 4 secs.

Whether the differences is important or not depends on what is being measured. In the paper you referenced, data were being combined from multiple sources measuring very small timing variations and using BJDs made the timing errors as small as possible.