Objects in the sky and relation to time
The only clocks and calendars our early ancestors had were the motions of objects in the sky. All of us pay some attention to the position of the Sun through the day. Few of us note the position of the Big Dipper as the sky darkens. If you do notice it and realize that its position shifts 15 degrees counterclockwise around Polaris each hour, it’s not a stretch to see the position of the Pointer Stars at the end of the bowl as the hand of a cosmic clock.
Even fewer of us know the patterns of the moon that can also act as a cosmic clock. Almost everyone does know the that moon’s phases gave us the month … and Monday (“Moon” day), but barely pause to notice our cosmic neighbor in the sky with anything more than, “Oh, there’s the moon.” I hope my last column did encourage you to watch the waxing crescent and gibbous phases between the clouds over the past two weeks. After the full moon last Friday, it is now in the waning gibbous phases that we generally see less of since the moon rises later and later at night.
There is a fairly straightforward relationship between the time of day and the rising, transit and setting times of the various phases. The “transit” time is when the moon (or any object) crosses our meridian, the imaginary line from due north up through the zenith and down to due south. We speak of the meridian in our times as a.m. stands for the Latin ante meridiem, meaning “before the meridian.
“These times are before the sun crosses the meridian, or transits, at noon.
The diagram shows the Earth as seen from above the North Pole with the Sun to the right. Since Tupper Lake is on the line from the center of the Earth to the sun, it’s noon. In the Canary Islands just west of the coast of Africa, it’s 3 p.m., while it’s 6 p.m. for Eastern Europeans and just after 6 a.m. for Hawaiians. The time of day is actually just our position relative to the Earth-sun line. Imagine the Earth in the diagram rotating on its axis while the times remain in space. In about another six hours, Hawaii will pass the noon position and Tupper Lake will pass the 6 p.m. position.
The different phases of the moon are also defined by the moon’s position relative to the Earth-sun line as also shown on the diagram. The outer ring shows the moon orbiting the Earth with its sunward face always bright. The inner ring shows the moon as it appears in the sky due to only the half facing Earth being visible to us. The times when those phases transit are shown just below them.
When the moon is between with the Earth and sun on that line, it’s the new moon. That means that we see the new moon transit at noon, with the sun. As the moon’s orbit takes it away from the sun to the east, we start to see the waxing crescent phases. Note how the middle of these aligns with the “3 p.m.” time. That means the moon will transit at 3 pm when it’s at that position. The first quarter moon, at the 6 p.m. position, will transit at that time. This applies throughout all the phases. The full moon always transits at midnight, the 3rd quarter transits at 6 a.m.
Those patterns aren’t too difficult to get if you have the diagram in mind. The rising and setting times, however, get more complicated. The new moon is easy, it rises and sets with the sun. For the others, a decent first approximation is that the moon rises six hours before it transits and sets about six hours after it transits. For the first quarter moon, that transits at 6 p.m., it rises about noon and sets at about midnight. The middle waxing gibbous phase that transits around 9 p.m., rises at about 3 p.m. and sets at about 3 a.m.
One can also use the angles to locate the sun, and thus know the time, when you see the moon. The new moon is aligned with the sun, the full moon is opposite the sun in the sky. If it’s transiting, it’s midnight. The quarter moons are 90 degrees from the sun with the lit side pointing the direction … the sun is 90 degrees right of the first quarter moon and 90 degrees left of the third quarter moon (in the Northern Hemisphere). The waxing phases are within 90 degrees of the sun and the gibbous phases between 90 degrees and 180 degrees from the sun. Using the “thickness” of the phase as an indication of the angle between the moon and sun can help you estimate the clock time every time you see the moon.
If you can get these patterns in your head, the moon’s phase and position can be your own personal cosmic clock. Many of our ancestors learned these patterns as young children and read the moon and stars as easily as we look up the time and date on our computers or phones.
For me, watching these patterns connects me to those ancient people. I watch the same moon going through the same patterns that Galileo, Ptolemy … and my own great grandparents did. Somehow it connects us and connects me more strongly to this amazing universe.
The astronomers of the Adirondack Public Observatory will be happy to share views of the Moon and other wonders of the night sky from our Roll-Off-Roof above Little Wolf Pond in Tupper Lake. We’re open to the public on the first and third Fridays of each month, weather permitting. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO at 518-359-6317 to talk with one of our astronomers. Observing starts about one half hour past Sunset.