Greetings, stargazers.
In the last couple of weeks, most of us have done more snow shoveling than sky watching, but I hope that hasn’t kept you from having a happy new year so far. At least this is a new year, according to the “modern,” Gregorian calendar from 1582. And as I change wall calendars to 2016, this is as good a time as any to discuss how astronomers like to record time. Something that is much more peculiar than you might think.
The Gregorian calendar was a modification of the Julian calendar. The one from Julius Caesar in 45 B.C. brought us leap years like we get to have in 2016. The change to the Gregorian caused a 16th-century disruption of 10 calendar days, so dual dating during the transition was necessary.
Believe it or not, some countries didn’t change calendars until the early 20th century, and some Eastern Orthodox churches that still use the Julian calendar are only now in the midst of the 12 days of Christmas. Settling on January as the first month of the year, rather than March, had a similar disruption on the way dates were recorded. How are we to know when anything really happened?
One quirk of the transition was the introduction of the Julian day to help with calculating conversions between the two calendars. Day zero was chosen to start at noon Jan. 1, 4713 B.C. That day was chosen because it happened to be a day when several periodic civil and astronomical cycles aligned and was also before any recorded historical event. Today is Julian day 2457402.
Also, rather than hours (converting by 24), minutes (converting by 60) and seconds (another 60), it is easier for computers to record the decimal fractions of days. If I observe an astronomical event at 7:05 p.m. Jan. 14, 2016, Durango time, I would convert that to Julian day 2457402.586806! This unwieldy-looking decimal number is like a metric system for the date of an event and is what astronomers might typically use to report an observation.
This number will keep incrementing without regard for years, leap years, months, day of the week or even time zones. That steady progression is something that even scientists beside astronomers can appreciate. So this 16th century aid for date conversion still gets used for recording astronomical events in 2016.
I don’t anticipate Julian days will ever come into common usage, and unless you are reporting your observations to one of many online data repositories, you don’t have to worry about making the conversion. Fortunately, computer programs can easily convert these decimal numbers to and from something more recognizable by people.
This month
Jupiter is now rising before midnight and will soon be a bright evening object. Venus is still a very bright morning star that was recently in a conjunction with the much more distant, and thus much dimmer, Saturn.
Mars is about halfway between Jupiter and Venus but is still much dimmer than the other two. Comet Catalina is still visible, but fading.
A telescope will really help with observing all these relatively bright, but very tiny, objects.
Charles Hakes is a visiting assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory. Email him at hakes_c@fortlewis.edu.
Useful links
Julian day: https://en.wikipedia.org/wiki/Julian_day
Julian date converter: http://aa.usno.navy.mil/data/docs/JulianDate.php
Astronomy picture of the day: http://apod.nasa.gov/apod/
An astronomer’s forecast for Durango: www.cleardarksky.com/c/DrngoCOkey.html?1
Old Fort Lewis Observatory: www.fortlewis.edu/observatory
Four Corners Stargazers: http://groups.google.com/group/Four-Corners-Stargazers
