Julian year (astronomy)

In astronomy, a Julian year (symbol: a or a_j) is a special way to measure time. It is defined as exactly 365.25 days of 86400 SI seconds each. This length comes from the average length of a year in the Julian calendar, which many Western societies used before switching to the Gregorian Calendar.

Even though it is called a "year," the Julian year in astronomy is used to measure how long something lasts, not to mark calendar years. It helps scientists keep track of time in a very exact way, which is important for studying stars, planets, and other objects in space. This way of measuring time is different from the years we use in our everyday lives.

Usage

The Julian year is not part of the International System of Units, but it is accepted by the International Astronomical Union for use in astronomy. Before 1984, astronomers used both the Julian year and the mean tropical year. The Julian year is very useful because it is always the same length, defined exactly in seconds, making it very accurate. It helps us measure both the sidereal year and the tropical year very closely. The Julian year is also important for defining the light-year, a unit used to measure very large distances.

Epochs

Main article: Epoch (astronomy)

In astronomy, an epoch is a specific moment in time that helps us know where stars and planets are. Because these objects move, we need to pick a point in time to measure from. We update this point about every 50 years.

Today, we use Julian epoch J2000.0. This is exactly midnight on January 1, 2000, in the Gregorian calendar. The word “Julian” in its name means we can count time in Julian years, which are 365.25 days each. For example, the next epoch, J2100.0, will be 36,525 days after J2000.0, also at midnight on January 1, 2100.

Because Julian years and Gregorian years are not exactly the same length, over many years the two will start to differ. In the next 1,000 years, the Gregorian calendar will lose seven days compared to Julian years. So, J3000.0 will actually be January 8, 3000, at midnight.

Julian calendar distinguished

Main article: Julian calendar

The Julian year is a way to measure time that is always the same length. It is not the same as the actual years in the Julian calendar, which changed in length over time. Astronomers do not give numbers to Julian years. Instead, they use the Gregorian calendar for events after October 15, 1582, and the Julian calendar for events before that date. Sometimes they use other local calendars if needed.

They also use Julian day numbers to keep track of time in a consistent way.

Julian day distinguished

Main article: Julian day

A Julian year is different from a Julian day, which is also used in astronomy. The Julian day is more properly called the Julian day number or JDN. It helps astronomers keep track of time without worrying about calendars, months, or years.

The Julian day number was created to make it easier to work with old dates from many different places. Astronomers started using it in the 1800s. It counts the number of days that have passed since a special starting date, called an epoch. This starting date is chosen to be before most historical records. To be even more exact, a specific time during a day is noted using UTC, with fractions of a day added for precision.