**What is Julian Date?**

**The International Astronomical Union** unites many astronomical societies from around the world under its roof. This union, which is a member of the International Science Council, is the official institution of the astronomical community.

**Julian Day**, a time measurement system, is included in the Julian date system. The Julian Day proposed by the International Astronomical Union for use in astronomical studies, BC. Presents the time interval of a day in fractions of days starting from January 1, 4713.

**How is Julian Date Calculated?**

Julian Day, BC. It presents the time interval of a day starting from Noon with Universal Time (UT) on Monday, January 1, 4713, in fractions of the day and the day. In other words, noon was chosen as the starting time of the day. The first day is considered to be the 0. **Julian Day**. In this way, the multiples of 7 always correspond to Monday. Negative values can also be used, but these values invalidate any history saved. While each day is expressed as an integer, any hour of the day is added to this integer as a fraction of the day. The date January 1, 4713 is the beginning. The days that come after this date have been counted consecutively and added to this number. These numbers are indicated in the astronomy annals as charts by years, months, and days. **The Julian Date**, abbreviated as JT, is the date determined by these numbers. The Julian Date of any date can be found in 2 ways:

The charts in the astronomy annals are checked.**The Julian Day** calculation formula is used.

The formula for calculating the Julian Day is as follows:

JT = 2415020 + 365 x (year – 1900) + N + L – 0.5

This formula; It gives the Julian Date that coincides with any date since 1990. Universal time is EZ = 0 in this calculation. N is the number of days after the new year, and L is the number of leap years between 1901 and the date to be calculated. In the formula; 2415020 refers to the **Julian Day **corresponding to January 1, 1900, and the amount 0.5 denotes the decimal equivalent of the half day resulting from the start of the Julian Day in the middle of the day.

The formula for finding the number N is as follows:

N = <275M / 9> -2 <(M + 9) / 12> + I-30

In this formula, M is the number of months, and I is the day of the month. Also <> indicates that the full part of the number in the brackets will be taken. For a leap year, the factor of 2 in the 2nd term is removed.

Correspondence of the decimal parts of Julian Date in hours, minutes and seconds:

0.1 = 2.4 hours or 144 minutes or 8640 seconds

0.01 = 0.24 hours or 14.4 minutes or 864 seconds

0.001 = 0.024 hours or 1.44 minutes or 86.4 seconds

0.0001 = 0.0024 hours or 0.144 minutes or 8.64 seconds

0.00001 = 0.00024 hours or 0.0144 minutes or 0.864 seconds.

**Julian Calendar**

The Julian Calendar is the most famous of the solar calendars. B.C. It was prepared by the Roman Emperor Jules Caesar (Jules Ceasar) in 46 BC and was used in the western world until the 16th century. It is also known as the Julius Calendar. It is considered to be the first version of the **Gregorian calendar**. Aiming to solve the confusion and problems in the calendars used in the past years, Caesar received help from the Alexandrian astronomer Sosigenes. With the suggestion of Sosigenes, this calendar was prepared based on the movements of the sun, not the moon. Aiming to correct seasonal shifts, Sosigenes calculated a year as 356.25 days. In this way, while the years that cannot be divided into 4 are 365 days, the increasing quarter days from these years were added to the 4th year and the leap year was increased to 366 days. In order for 1 year to be 12 months, leap years are arranged to be 6 months 30 days, and the other 6 months 31 days. In non-residual years, 1 day is removed from the last month of the year. At that time, New Year’s Eve was in March. Therefore, February, which is the last month of the year, has been reduced to 30 days in leap years and 29 days in other years. **Caesar** wanted to immortalize his name as the organizer of the calendar and changed the name of July to July.

The arrangements made in the calendar after the death of **Caesar** could not be implemented properly. The fact that Pontifeksler, who made arrangements in the calendar, made a leap year application in 3 years, caused confusion again. During the 40 years of application in this way, there has been 3 days of slippage. Roman Emperor Augustus, BC. In the 8th year, it corrected this shift by stopping the leap year application for 12 years. He also changed the name of August to his own name, Augustus. In the regulations made; It was taken 1 day from February, and added to August, after July 31 and August took 30 days. In this way, February took 29 days in leap years and 28 days in other years. Julian Calendar, BC. It was used from the 46th to the 16th century.

Leap year practice was applied in **Julian Calendar **for the first time in history. As a result of a small difference in this calculation, a 1-day shift occurred approximately every 128 years. Due to the confusion created by this shift, the Julian Calendar was abandoned in the 16th century and the Gregorian Calendar was adopted.