Julian day facts for kids

The Julian day or Julian day number (JDN) is a way to count days. It's the number of days that have passed since a special starting point called an epoch. This starting point was set at noon Universal Time (UT) on Monday, January 1, 4713 BC, in the Julian calendar. That specific day, from noon to noon, is called Julian day 0.

This system is super useful for astronomers and scientists because it gives every single day a unique number. It makes it easy to figure out how much time has passed between any two dates, even if they are thousands of years apart!

What is a Julian Day Number (JDN)?

A Julian Day Number (JDN) is a whole number that counts each day. For example, if a JDN is 0, it means it's the very first day of this special counting system. If the JDN is 1, it's the next day, and so on.

• The JDN changes at noon UT, not at midnight. This is different from how we usually think of days changing.
• If a JDN can be divided by 7 with no remainder, that day is a Monday. This helps keep track of the day of the week.
• Sometimes, negative JDNs are used for dates even before recorded history.

For example, right now, the JDN is 2460845 (update). If you divide this JDN by 7, the remainder is 2. This remainder tells you the day of the week, where 0 means Monday.

What is a Julian Date (JD)?

The Julian date (JD) is like the Julian day number, but it's even more precise. It's a continuous count of days and also includes fractions of a day. This means it can tell you the exact time of day, not just the day itself.

• The JD is currently 2460845.8152778.
• The whole number part of the Julian date (its floor) gives you the Julian day number.
• The part after the decimal point tells you the time of day since noon UT. For example, 0.5 means midnight UT.
• Computers can store Julian dates with amazing accuracy, often down to about 1 millisecond (that's one-thousandth of a second!).

For example, a Julian date of 2454115.05486 means the date and time is Sunday, January 14, 2007, at 13:18:59.9 (which is 1:18 PM and 59.9 seconds).

Understanding the Decimal Parts

The decimal part of a Julian date helps you know the exact time. Here's what some of those fractions mean:

• 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

Why are Julian Days Used?

Almost 2.5 million Julian days have passed since the counting started! Julian day 2,400,000 was on November 16, 1858. Julian date 2,500,000.0 will happen on August 31, 2132, at noon UT.

The Julian day system is like a very simple calendar where each date is just a number. This is super helpful for:

• Reference: Having a single, continuous count for all dates in history.
• Computations: It makes it easy to calculate the exact time between any two dates by simply subtracting their Julian day numbers.
• Conversions: It helps convert dates between different calendars, like the Julian calendar and the Gregorian calendar we use today.

Astronomers created the Julian day system to have one standard way of dating events. This helped them work with different calendars and historical records more easily. It's important to remember that the Julian day and Julian date are not directly related to the Julian calendar itself, even though they share a similar name. They are just a way to count days continuously.

See also

In Spanish: Fecha juliana para niños