The Julian date
 
The irregularities in the present calendar (unequalmonths, days of the week having different dates from year to year) and the changes from the Julian to the Gregorian calendar make it difficult to compare lengths of time between observations made many years apart. Again, in the observations of variable stars, it is useful to be able to say that the moment of observation occurred so many days and fractions of a day after a definite epoch. The system of Julian Day Numbers was, therefore, introduced to reduce computational labour in such problems and avoid ambiguity. January 1st of the year 4713 BC was chosen as the starting date, time being measured from that epoch (mean noon on January 1st, 4713 BC) by the number of days that have elapsed since then. The Julian Date is given for every day of the year in The Astronomical Almanac. Tables also exist for finding the Julian Date for any day in any year.
For example, the Julian Date for June 24th, 2000, is 245 1719·5 when June 24th begins; again the time of an observation made on June 24th, 1962, at 18h UT is JD 245 1720·25.
Time may also be measured in Julian Centuries, each containing exactly 36 525 days. Orbital data for artificial Earth satellites are often referred to epochs expressed in Modified Julian Date Numbers in which the zero point in this system is 17·0 November, 1858. Hence, Modified Julian Date = Julian Date − 240 0000·5 days.
For some astronomical observations, the fact that the Earth is in orbit about the Sun causes a difficulty when accurate timings of events are required. Observed delays and advances relative to predicted times of satellite eclipses by Jupiter were noted by Roemer, the effect turning out to be a milestone in connection with the determination of the velocity of light. Resulting from the Earth’s motion about the Sun, the effect is also a problem with timed measurements related to any celestial body. For example, for a star lying close to the ecliptic, the path length that the light has to travel will change from night to night according to the Earth’s orbital motion. Over a six-month period, the difference in timings can be as much as ∼ 16 minutes, this being the time it takes for light to cross the diameter of the Earth’s orbit. For a star nearer to the ecliptic pole, the path length variation will be much smaller.
In order to compensate for the effects, times of astronomical observations are sometimes expressed in terms of an Heliocentric Julian Date or HJD, the Julian Date of the record transposed to a timing that would have been obtained at the centre of the Sun. As the effect is related to the Earth’s orbit, it will be appreciated that the correction formula will involve the ecliptic coordinates of the observed object and those of the Sun. Quite simply, the correction conversion formula may be written as
HJD = JD − 0·d0058 cosβ cos( − λ) (1)
where  is the solar longitude on the ecliptic and β, λ are the source’s ecliptic latitude and longitude respectively.