Timekeeping in Astronomy

"Captian's log, stardate 1513.1. Our position: Orbiting planet M-113. Onboard the Enterprise: Mr. Spock temporarily in command. On the planet: the ruins of an ancient and long dead civilization. Ship surgeon McCoy and myself are now beaming down to the planet's surface. Our mission: routine medical examination of archaeologist Robert Crater and his wife, Nancy. Routine, but for the fact that Nancy Crater is that one woman in Dr. McCoy's past."

 - Opening of Star Trek: The Original Series, Season 1, Episode 1; "The Man Trap"

If you're curious like me, you'd have noticed that the Captain's log from Star Trek used some form of strange new system of measuring time. What does stardate 1513.1 represent? It turns out, it doesn't really mean much. It was just another way of making the show look futuristic. From Wikipedia:

"We invented "Stardate" to avoid continually mentioning Star Trek's century (actually, about two hundred years from now), and getting into arguments about whether this or that would have developed by then. Pick any combination of four numbers plus a percentage point and use it as your story's stardate."

But interestingly, the convention used by astronomers to keep time is very similar sounding to the stardate system seen in Star Trek. Astronomers use what is called a Julian Date whenever the time and date of a measurement needs to be mentioned.

Calendars have a long and messy history. The calendar system that is in use around the world today is called the Gregorian Calendar. People only started using this calendar in the late 1500s. Before that there was something called the Julian Calendar which was introduced by Julius Caesar in 45 BC. These are two big calendars. But there were also a lot of other calendars in use by different countries and cultures. A look at calendar systems in wikipedia shows a mind boggling and confusing list of widely varying systems. Some are similar, some are wildly incompatible. Also, due to the fact that the time taken by the earth to go around the sun is not an exact integer multiple of a day, errors inevitably arise when measuring time in years or months. Astronomers needed a standardized method of referring to time for astronomical observations. This was the motivation behind the adoption of the Julian Date.

A Julian Day Number is an integer that represents the number of days that have passed since 1st January 4713 BC in the proleptic Julian Calendar or 24th November 4714 BC in the proleptic Gregorian Calendar. (NOTE: A preleptic calendar is a calendar obtained by extending a calendar system backwards in time to date before AD 4)

The Julian Date of any point in time is the Julian Day Number of the previous day along with a decimal number that represents the fraction of the next day that has passed.

For example, the 1st of January, 2014 1800 hrs is represented by a Julian Date of $2456659.25$

I hope everyone has a wonderful time celebrating another complete revolution of the Earth around the Sun. An exciting year lies ahead!