Sundials in space: sundial on a Mars Rover

Parapegma for Astronomical Twilight at Salt Lake City, Utah

Revised 6/26/2006

A parapegma is a table of rising constellations after sunset. In Greek and Roman times, persons would memorize their parapegma as common knowledge - much like we would memorize multiplication tables today. Using the table of rising constellations at sunset and knowing the number of seasonal hours at night for given month, it is possible to tell time at night. For example, in June Sagittarius is the rising zodiac constellation at astronomical twilight. There are 45 minutes or 3/4 of a standard hour to a seasonal hour. In 6 seasonal hours - or (6 * .75) 4 1/2 standard hours, Sagittarius will transit. That will occur at approximately 2:30 am in the morning.

The columns labelled "Zenith Constellation", "Transiting Equator", and "Transiting Zodiac" are useful for observation planning. Those are the constellations that will be most favorably positioned for viewing. Use this table to enter your guidebooks that are organized by constellation name. It is useful to look at the following month in the table when planning a night. Two hours after astronomical twilight, transiting constellations will be the ones listed in the following month.

In modern amateur practice, an analogous table is Table 1 - Approximate Sidereal Time in Volume One of Burnham's Celestial Handbook (p. 59). Such planning is normally done with a planisphere - an inexpensive disk that can be purchased at your local planetarium.

Table 1 - Parapegma for Astronomical Twilight at Salt Lake City, Utah
Date Astronomical Twilight Time Ecliptic Transit Altitude Seasonal Hour Due East Rising Rising Zodiac Zenith Constellation Transit Right Ascension J2000 Transiting Equator Transiting Zodiac Setting Zodiac Due West Setting Season Overnight Temp F Percent Cloudy
3/1 19:50_MST 72 1:03 Vir Vir Cep 5h29m Ori Gem Psc Psc Winter 31 52
4/1 21:27_MDT 67 0:56 Vir Vir Lyn 8h41m Hyd Cnc Psc Cet Spring 38 47
5/1 22:09_MDT 52 0:50 Oph Sco Uma 11h21m Leo Leo Tau Ori Spring 46 39
6/1 22:56_MDT 41 0:45 Aql Sgr Boo 14h10m Vir Vir Gem Hyd Spring 55 20
7/1 23:08_MDT 27 0:44 Aqr Cap Her 16h21m Oph Sco Cnc Sex Summer 64 13
8/1 22:33_MDT 45 0:48 Psc Psc Her 17h47m Oph Sgr Vir Vir Summer 62 13
9/1 21:36_MDT 44 0:54 Psc Psc Lyr 18h53m Aql Sgr Vir Vir Summer 51 17
10/1 20:41_MDT 44 1:01 Cet Psc Cyg 19h56m Aql Cap Lib Vir Fall 40 29
11/1 18:56_MST 34 1:07 Cet Tau Cyg 21h09m Aqr Cap Sco Ser Fall 31 47
12/1 18:36_MST 42 1:12 Ori Gem Lac 22h51m Psc Aqr Sgr Oph Fall 22 58
1/1 18:49_MST 56 1:13 Mon Cnc And 1h07m Psc Psc Cap Aql Winter 19 61
2/1 19:19_MST 69 1:09 Hyd Leo Per 3h39m Ari Tau Aqr Aqr Winter 25 57

Prepared by and report errors or broken links to: K. Fisher 6/2006