Julian day is a concept you might occasionally encounter in SQL code or database operations, particularly when working with date and time functions. While it may seem esoteric at first, understanding Julian day can be incredibly useful for handling date calculations, especially in fields like astronomy, data analysis, and historical research.

This article looks at the origins, calculations, and practical applications of Julian day, including examples of converting between Julian day and other date formats in SQL.

What is Julian Day?

Julian day, often abbreviated as JD, is a continuous count of days since the beginning of the Julian period on January 1, 4713 BCE (proleptic Julian calendar). It was introduced by Joseph Scaliger in the 16th century to provide a single continuous system of dates, avoiding the complications of calendar changes and the discontinuities in different systems.

Julian day is primarily used in astronomy, computing, and other fields where consistent timekeeping is crucial. Its simplicity lies in its numeric representation, which eliminates the need for complex date manipulations inherent in calendar systems.

The Julian day starts at noon Universal Time (UT), making it convenient for astronomers who often work through the night. This means that the Julian day number changes at noon rather than midnight.

How Julian Day is Calculated

The Julian day is calculated as the total number of days elapsed since the starting epoch of the Julian Period (January 1, 4713 BCE, at noon UT). Fractional days are expressed in decimal format, allowing for precise time representation.

For example:

• January 1, 2000, at noon UT is Julian Day 2,451,545.0.
• January 1, 2000, at midnight UT is Julian Day 2,451,544.5.

Converting Between Julian Day and Gregorian Date in SQL

Below are two examples that use SQLite to demonstrate conversions between Julian day and Gregorian datetime.

Convert Gregorian Date to Julian Day

SQLite has a julianday() function, which we can use to convert a Gregorian date to its Julian day equivalent:

SELECT julianday('2000-03-01 00:00:00') AS julian_day;

This query returns:

julian_day
----------
2451604.5

Convert Julian Day to Gregorian Date

SQLite has a datetime() function that we can use to return a Julian day value to a datetime value:

SELECT datetime(julianday(2451604.5)) AS gregorian_date;

This query returns:
2000-03-01 00:00:00

The way you deal with Julian day values will depend on your RDBMS. Here are a couple of PostgreSQL examples:

• Convert a Julian Day to a Date in PostgreSQL
• Convert a Date to Julian Day in PostgreSQL

Applications of Julian Day

Astronomy

Astronomers use Julian day to calculate celestial events like eclipses and planetary positions. The simplicity of Julian day calculations helps in accurately predicting events over millennia.

Computing

Julian days provide a unified way to manage dates across systems, avoiding issues with varying calendar standards.

Historical Research

Historians use Julian day to reconcile dates from different calendar systems when analyzing ancient texts or records. They also use Julian day when no such year was given in the historical record, or when the year given by previous historians was incorrect.

Limitations of Julian Day

• Time Zones: Julian day calculations are inherently in Universal Time (UT). Conversions to local time require additional steps.
• Accuracy: The method assumes a uniform progression of days and does not account for Earth’s slowing rotation or leap seconds.

Summary

Julian day is a versatile and straightforward system for tracking time, offering significant advantages in fields that require precise date computations. While its primary use is in astronomy, its applications span various domains, proving its enduring relevance.

Understanding how to compute, convert, and apply Julian day enhances our ability to work with time-dependent systems efficiently. We can use SQL to convert to and from Julian day values, but the exact method we use will depend on our RDBMS.