THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian Date: 2459397.18
2020-2021: CXLVI
THE DAILY ASTRONOMER
Thursday, July 1, 2021
July 2021 Night Sky Calendar Part I
Happy New Year!
We offer these felicitations because the new fiscal year has started here at the University. As our planetarium planetarium is a miniscule outpost within this grand academic fiefdom, we naturally abide by the same calendar. Hence the happy new year bit. (The new DA school year, conversely, will begin on September 1.) As is true with every other month since months began with the Earth-splintering/moon creating Theia impact, this month will prove to be breath-stoppingly exciting and shall offer a rich abundance of retinae-delighting eye confections.
And, as is our wont, we're dividing the calendar into parts.
Let's go!
THURSDAY, JULY 1: LAST QUARTER MOON
What do you mean we're starting the fiscal year with a cough and whimper? Last quarter moons are far more enthralling that many people believe. While the full moon garners 98.7% of all the attention, the quarter moons are beguilingly beautiful in their own rite. The best time to observe the quarter moon is when it is rising. (The July 2021 last quarter moon rises at 12:32 a.m.) The ascending moon is steeped in a rich ember glow due to atmospheric light scattering effects.
We advise all sky watchers to venture outside after midnight to observe the quarter moon as it peeks above the eastern horizon while progressing from deep ember to tangerine and then yellow-white. A perfect way to usher in the witching hour.
SUNDAY, JULY 4: MERCURY AT GREATEST WESTERN ELONGATION
Are you looking for Mercury? Well, any Mercury admirer would be well advised to venture out this morning to find the elusive little planet. The first world attains its greatest western elongation today and will be 22 degrees from the Sun! Mercury will rise today at 3:51 a.m, about ten minutes after the onset of nautical twilight. At magnitude 0.6, Mercury will be slightly dimmer than Betelgeuse and so will be bright enough to observe against the intensifying twilight.
MONDAY, JULY 5: EARTH AT APHELION
Why do we have seasons? Some people believe that the seasons relate to the Earth's changing distance from the Sun. Summer is hotter because Earth is closer to the Sun. Winter is colder because we are farther from the Sun. Though wholly logical, that notion is completely wrong. We have seasons because Earth is tilted on its axis. In the summer, our hemisphere is directed toward the Sun; in the winter, it is oriented away from it. Distance has little to no effect on our weather at all. In fact, Earth is farthest from the Sun (aphelion) in early July and closest to the Sun (perihelion) in early January.
This diagram shows Earth's perihelion and aphelion positions. The orbital eccentricity (departure from circularity) is highly exaggerated! The difference between the perihelion and aphelion distances amounts to little more than 3 million miles. (Earth's orbit looks almost circular.)
Today Earth is about 151 million kilometers from the Sun. After reaching aphelion, Earth will start to move toward the Sun until reaching perihelion on January 4, 2022
TUESDAY, JULY 6: MOON NEAR PLEIADES
We'll see the waning crescent moon (10% illuminated) apparently close to the Pleiades Star Cluster low in the eastern pre-dawn sky. The Pleiades resembles a faint light smudge poised on the shoulder of Taurus the Bull.
FRIDAY, JULY 9: NEW MOON
The beginning of lunation cycle 1219.
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