This column is in response to a reader’s question about the ways the moon may have been used to tell time by early people. Lacking even the simplest clocks, the ancients had to be clever in using heavenly bodies to determine the time.
The sun allowed them to tell the time of year if they noted the place along the horizon where it rose or set. All through the winter and spring, the sun rises slightly to the north of its previous rising point (same statement applies to setting).
When the sun rises or sets farthest to the north, that marks the first day of summer. Then all through the summer and fall, the sun rises or sets slightly southward of its position of the previous day. Then the day when the sun rises or sets farthest south is the first day of winter.
When the sun rises or sets exactly half way between these two extremes, we have the first day of spring or fall. To use the sun to tell the time of day, you can use a sundial. In mid day (about 1 p.m. Eastern Daylight Time), shadows are shortest and point north.
The moon also has some possibilities for time keeping. The moon was the original basis of our months. Both the Jewish and Islamic months still begin with the first sighting of the crescent moon at dusk from their holiest place (Jerusalem for the Jews and Mecca for Muslims).
Because lunar months don’t fit neatly into a year of the seasons, Jews add a 13th month to their calendar seven times in a span of 19 years. This keeps their feasts such as Rosh Hashanah, Passover, etc., in line with the seasons. Christians set the date of Easter by a rule that Easter falls on the Sunday following the first full moon of spring.
To use the moon to tell the time of the year, one could notice just the full moon and note what star group it falls in. The full moon appears directly opposite the sun so the full moon group would be in the opposite direction as the group the sun appears in.
For instance, at this time of the year (early May), the sun appears in front of the stars of Aries. So six months from now, the full moon will appear in front of Aries in early November. The Zodiac progression for the full moons would be: January-Gemini, February-Cancer, March-Leo, April-Virgo, May-Libra, June-Scorpius, July-Sagittarius, August-Capricorn, September-Aquarius, October-Pisces, November-Aries and December-Taurus.
There are two problems in using the full moon for telling the time of the year: You must know the actual evening of the full moon (It’s not that easy to be sure) and full moon dates change by about 11 days from year to year. Next Saturday evening, the moon is full, shining among the stars of Libra.
You could use some of the moon’s phases to tell the time of the night. The full moon appears opposite the sun, so when the full moon is highest in the south, the time is local midnight (1 a.m. for Eastern Daylight Time).
When the moon appears half full, it will be setting at local midnight (first quarter phase) or rising at local midnight (third quarter phase). A waxing moon that is one quarter illuminated (crescent in p.m. hours) will set about three hours after sunset. Likewise, a waning moon that is one quarter illuminated (crescent in a.m. hours) will rise about three hours before sunrise.
New presentation
Our May program at the Frostburg State is “Earth’s Greatest Hits” with free presentations today, May 10 and May 17 at 4 p.m. and 7 p.m. This program will focus on the nearer bright stars (within about 100 light years). From these stars, the Earth would appear at different times in the 20th century. (A star that is 55 light years away would see the Earth as it was in the 1950s, at the onset of Rock and Roll.)
There will also be an informal tour of the current evening sky with questions from the audience encouraged. Free star charts and a summary of this program are available to our audience. These programs last about 45 minutes. Following the program, visitors are invited to tour the Science Discovery Center in the Compton Center nearby.
The Planetarium is in Tawes 302, just off the front lobby. Tawes Hall is a medium sized building near the Performing Arts Center, the Compton Center, the Lane University Center and the FSU Clock Tower. Please arrive a few minutes early as seating is limited.
Call (301) 687-7799 to request a Planetarium/Discovery Center Bookmark, sent free to you through the mail.
Bob Doyle invites reader’s comments or questions: Call above phone number (leave voice mail) or email him at rdoyle@frostburg.edu .
Bob Doyle - Astronomy
Using moon to keep time is old tradition
- Bob Doyle - Astronomy
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