Julian Date 2460000: A Look Back & a Look Ahead

A significant milestone in astronomical timekeeping takes place this month that may pass unnoticed. On February 24, at 7:00am Eastern Standard Time (12:00 Coordinated Universal Time), the Julian Date will be 2460000.0. 

The Julian Date is a system of measuring time in decimal days as opposed to varying quantities of years, months, and days. This facilitates calculations of time intervals between astronomical events by a simple subtraction of one Julian day from another. This is useful for measuring periodic events such as variable stars, where it is most commonly used. 

Julian Dates roll over at 12 hours Coordinated Universal Time. This gives night observers in Europe, Africa, most of the Americas, and parts of Asia the ability to log observations under a single Julian day, rather than two separate dates, as is the case when using Gegorian calendar dates.

So, what exactly happened 2460000 days ago that set the zero point for this method of timekeeping? If we subtract 2460000 days from February 24th, we get January 1, 4713 BC. Nothing notable occurred on that date, that we know of, so why set that as the zero point?

This is just the number of days since the start of the current Julian Period, which is defined as the product of three time cycles that today are rather obscure: the 28-year solar cycle, the 19-year Metonic (lunar) cycle, and the 15-year Indiction cycle, for a total of 7890 years.

The 28 year solar cycle is derived from the interval of years before the calendar dates occur on the same days of the week as the previous cycle, accounting for leap years, but omitting century leap years.

The Metonic cycle repeats when the lunar phases occur on the same days of the year, which is equivalent to 235 synodic cycles, which happens to be almost exactly 19 years.

The Indiction cycle is a Roman-era 15-year interval used for tax assessment purposes. It was last used in medieval Europe, and would have been familiar to contemporaries of Joseph Justus Scaliger, who first proposed the Julian Period in 1583. When the beginning of all three cycles coincided, this defined the beginning of the epoch, which was 4713 BC. This was determined to be sufficiently far in the past, that any event of recorded history could be assigned a positive date.

This brings us to February 24, 2023. While there is nothing significant about the number 2460000, it is worth noting that a 10,000 day rollover occurs about every 27.4 years, so it is a little more significant than crossing a decade rollover in our Gregorian calendar.

Since we are talking about this mainly in reference to astronomical time markers, let’s consider where we were at Julian Date 2450000, and what has occurred since.

The calendar date on 2450000 was 12:00:00 UT on October 9, 1995.

The exoplanet revolution had just begun. 51 Pegasi, the first extrasolar planet orbiting a main sequence star to be discovered, was found just 3 days before (JD 2444997). 

Hubble Space Telescope’s famous “Pillars of Creation” image was released November 2, 1995 (JD 2450023).

Our Solar System was known to have nine planets, but the discovery of other trans-Neptunian objects had begun just three years earlier, with 1992QB1 on August 30 (JD 2448864).

The Keck 1 telescope had recently become the largest operational telescope in the world. Keck 2 would come online a year later.

After a 5-year journey, NASA’s Galileo probe, the first spacecraft to visit the planet since Voyager 2 in 1979, arrived at Jupiter on December 7, 1995 (JD 2450058). The mission would last for another 8 years.

Our last contact with the Pioneer 11 probe occurred on September 30, 1995 (JD 2449990). 

The NASA/ESA Solar and Heliospheric Observatory (SOHO) was launched on December 5, 1995 (JD 2450053). This indispensable solar observatory is still operational today.

The next 10,000-day interval rolls over on July 12, 2050. What fascinating astronomical discoveries will we be able to look back upon when we arrive at that date?

Looking farther ahead, the next 100,000 day rollover, JD 2500000, occurs on August 31, 2132. By then, we will have witnessed the next return of Halley’s Comet, on July 28, 2061 (JD 2474033) and will be only 2 years from the subsequent one on March 27, 2134 (JD 2500572). On May 1, 2079 (JD 2480519), Seagrave Observatory will have experienced its first total solar eclipse since 1925, and the next pair of Venus transits on December 11, 2117 (JD 2494622) and December 8, 2125 (JD 2497542) will have passed.

What other remarkable discoveries and deeper understanding of the universe will we come to know in that time? Only time will tell. In the meantime, enjoy the new 10k Julian Day,