Decimal fraction in Julian Date

Julian Day is a convenient way to count continuously days since the beginning of a specific time (noon of January 1, 4713 BC). This was proposed by Joseph Scaliger in 1583, as the beginning of the Julian Period, which epoch started when the three calendar cycles were together in their first year (4713 BC, preceding all historical dates).
[ Julian Period = 15 (indiction cycle) × 19 (Metonic cycle) × 28 (Solar cycle) = 7980 years ]

There are many online and offline tools to calculate the Julian Days. It is frequent though to use the fractional part of the day also, leading to a decimal fraction added at the Julian Day (becoming then the Julian Date – JD). It is not difficult to calculate the fractional part, as it is just a part of the whole day. But there is a small caveat, as the JD is calculated from the noon and not the midnight that we have get used with. So, when calculating an example I was using a time like 23h and 44m and I was getting as a result 0.9889, while other tools were giving the value 0.4889 ! It is logical to think that 23h and 44m is close to a whole day, IF you count from the midnight of the previous night. But just in the middle of the day IF you count from noon. So, by subtracting 0.5 (half day) you can go to the normal Julian Date and get the right decimal part.

Finally, in order to calculate the fractional part of the JD you can use the following formula:

JDfrac = [ hh/24 + mm/(24*60) + ss.s/(24*3600) ] – 0.5

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