Astronomical Calendar 2018, Instar 3

Here is another scene with pairs of planets, Mars and Jupiter high, Mercury and Saturn down low, tomorrow morning.

Mercury is closest south of Saturn at 7 by Universal Time on January 13, which for America was several hours earlier but below the horizon.

The scene has been added to “Astronomical Calendar 2018,” accessible through the tab at top.

Also added are some new categories of events, such as conjunctions involving the Moon, and “trios” – occasions when three bodies appear close together.  One of these (Moon, Mercury, and Saturn) will be on Jan. 15, though again down close to the pre-dawn horizon.

Another change I’ve made is to the Julian dates, at the left of each entry.  Astronomers like Julian dates because they enable subtracting to find spans of time between events, but mainly because all calculations are done with them, converting to unwieldy year-month-day-minute-second only to express final results.  (It’s much like the difference between using the decimal system and using pounds-shillings-pence or miles-yards-feet-inches.)

But, yes, Julian dates look intimidating, and they are seven-digit numbers. with more space taken up by three fractional digits if you want to carry them to roughly the precision of minutes, of which there are 1440 in a day.

So what I’ve done is chop off the leftmost three digits.  These are always 245 (between 1995 and 2023).  This is what is done, to save space, in some of the tables in the ponderous official Astronomical Almanac.  Of course subtraction between the beheaded Julian dates still works.

It’s just occurred to me to wonder when it was that Julian dates became 7-digit numbers, that is, reached the millions.  The smallish program I’ve built myself for answering miscellaneous questions gives -1975, that is, 1976 BC, on November 7 by the Gregorian calendar.  That was back in the Middle Kingdom of Egypt.

More will be added to “Astronomical Calendar 2018,” and you have made some kind suggestions.  I’m thinking about them, though I shall not be going to the lengths of the last and fullest printed Astronomical Calendar (for 2016, a few copies of which are still available).

No one has yet asked what I mean by using the word “instar.”  If anyone does…