THE SOUTHWORTH PLANETARIUM
70 Falmouth Street Portland, Maine 04103
(207) 780-4249 www.usm.maine.edu/southworth-planetarium
43.67° N 70.27° W
Founded January 1970
2022-2023: CVII
Sunrise: 5:01 a.m.
Sunset: 8:28 p.m.
Civil twilight begins: 4:25 a.m.
Civil twilight ends: 9:04 p.m.
Sun's host constellation: Gemini the Twins
Lunar phase: Waxing Gibbous
Moon rise: 2:13 p.m.
Moon set: 1:20 a.m. (6/28/2023)
Moon set: 1:20 a.m. (6/28/2023)
Julian date: 2460123.29
"I love criticism just so long as it's unqualified praise."
-Noel Coward
THE DAILY ASTRONOMER
Tuesday, June 27, 2023
What a Day!
Take it from the shades of the medieval geocentrists whose lachrymose spirits now float around a sun-centered orbit: astronomy complicates everything. We're not just referring to the space-time twisting black holes or the discorientingly complex formulae that enable us to know when Mars will next be occulted by the eclipsed moon. Even the simplest aspects of physical reality aren't as straightforward as they would have been had the Universe been governed by an out-to-lunch planetarium director who just wants the world to be happy.
Case in point: the day. How does one define it?
Simple: the amount of time required for Earth to complete one orbit on its axis. Well, how can we determine when one of those periods has elapsed? We're going to pretend that all clocks, calendars, graduated candles, water drip devices and all the other time-keeping mechanisms have been destroyed, leaving us with nothing except the sky. Now what?
Oh, that's simple: let's first project a luminous meridian onto the sky. A meridian is an arc that extends from due north to due south and passes through the zenith, the point directly overhead.
A little shadow person standing on the ground with the meridian passing overhead. Objects not drawn to scale. (Image credit: Wikipedia)
Next, let's watch the Sun. We'll wait until it crosses the meridian, which happens around noontime, but rarely at noon. The next day we watch and the Sun crosses the meridian almost exactly 24 hours later. Spiffing! The day is 24 hours long.
Or, well, is it?
We notice that a given star, let's say Mintaka, the western most of Orion's belt stars, crosses the meridian and then 23 hours, 56 minutes later, it crosses the meridian again! A four minute difference!
So, does Earth complete one turn every 23 hours, 56 minutes or every 24 hours? The answer is the former: Earth requires 23 hours and 56 minutes to turn once on its axis. The Sun appears to cross the meridian every 24 hours because our planet is revolving around it. During the time of one Earth rotation, Earth has advanced in its orbit by about one degree. Consequently, Earth has to turn by one degree in order for the Sun to return to the position it occupied the previous day. One degree equals 4 minutes, hence the discrepancy.
The time period separating successive passages of the Sun across the meridian is called a solar day and lasts 24 hours.
The time period separating successive passes of a star or, more correctly the point of the vernal equinox, across the meridian is called a sidereal day and lasts 23 hours and 56 minutes.
So, the next time you wish someone a good day, be sure to specify the day type. Happy Sidereal Day has such a good ring to it.
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