70 Falmouth Street Portland, Maine 04103
(207) 780-4249 usm.maine.edu/planet
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
2021-2022: V
"It's not what you look at that matters, it's what you see."
-Henry David Thoreau
THE DAILY ASTRONOMER
Wednesday, September 8, 2021
September 2021 Night Sky Calendar Part III
WEDNESDAY, SEPTEMBER 22: AUTUMNAL EQUINOX (GOLD EVENT!!!)
At 3:20 p.m. today astronomical autumn begins in the northern hemisphere while astronomical spring begins in the southern hemisphere. Today the Sun will rise due east and set due west. (The same will be true on the vernal equinox in March.)
The four seasonal dates for 2021 are as follows:
- SPRING March 20, 2021
- SUMMER June 20, 2021
- AUTUMN September 22, 2021
- WINTER December 21, 2021
Notice something about that list? The Autumnal equinox doesn't occur on the 20th or 21st. That is not unusual. The Autumnal equinox hasn't occurred on September 21st for more than a thousand years.* Why does the autumnal equinox occur on a later date in September than the other seasonal dates occur in their respective months? Simple. Astronomical summer is the longest season in the northern hemisphere. To understand why, we must discuss the reason we experience seasons.
We have seasons because Earth is tilted by about 23.5 degrees relative to the vertical. As Earth revolves around the Sun, the Northern Hemisphere is alternately oriented toward and then away from the Sun. Consequently, the Sun's altitude varies with the changing orientation: higher in the spring and summer; lower in the autumn and winter. During the spring and summer, the Sun remains above the horizon for more than twelve hours and its higher angle enables it to heat us more efficiently than it does in the autumn and winter.
If Earth's orbit were perfectly circular, our planet would require the same amount of time to progress from one seasonal point to another. However, the orbit is elliptical, so Earth's distance from the Sun varies continuously and so, too, does its orbital speed. (The closer a planet moves toward the Sun, the faster its orbital velocity will be.) Earth is closest to the Sun (perihelion) between January 1-4 and at its greatest distance (aphelion) around July 2-4. As Earth is closest to the Sun during winter, it is moving fastest along its orbit. In the summer, Earth is farther away and therefore its orbital speed is slower. Consequently, the time our planet requires to proceed from the winter solstice point to the vernal equinox is almost 89 days (88.9). The time that elapses between the summer solstice and autumnal equinox is 93.6 days. Our summer is more than four days longer than our summer.
The season durations:
Winter 88.9 days
Spring 92.8 days
Summer 93.6 days
Autumn 89.85 days
Just a reminder that we are referring to astronomical winter, not meteorological winter, which is interminable here along the nation's northern tier. We all know that autumn's inception heralds winter's imminent arrival. Of course, the less said about that, the better.
Happy Fall all!
THURSDAY, SEPTEMBER 23: A QUICK ZODIAC TRIP
As we're just beginning another school year, we wanted to review the "zodiac concept" again. The zodiac refers to the thirteen constellations through which the Sun appears to move throughout the year. This apparent motion is illusory, of course, because Earth is revolving around the Sun. Consequently, the Sun appears to migrate along a path in the sky called the ecliptic. This ecliptic passes through thirteen constellations.
The zodiac constellations -and the date range of the Sun's passage through them- are listed below. We'll start with the Sun's current host constellation, Virgo
- Virgo the Maiden September 16 - October 30
- Libra the Scales October 31 - November 22
- Scorpius the Scorpion November 23 - November 29
- Ophiuchus the Serpent Charmer November 30 - December 17
- Sagittarius the Archer December 18 - January 18
- Capricornus the Sea goat January 19 - February 15
- Aquarius the Water Bearer February 16 - March 11
- Pisces the Fish March 12 - April 18
- Aries the Ram April 19 - May 13
- Taurus the Bull May 14 - June 19
- Gemini the Twins June 20 - July 20
- Cancer the Crab July 21 - August 9
- Leo the Lion August 10 - September 15
A few brief notes pertaining to the differences between the astrological and astronomical zodiacs. First, the astronomical zodiac includes Ophiuchus the Serpent Charmer, which is not recognized in the astrological zodiac. Secondly, the Sun's duration within each constellation differs. For instance, the Sun only requires only a week to move through Scorpius, but remains within Virgo for a month and a half. The astrological zodiac assigns each constellation approximately 30 days. Finally, the dates are different. As an example, the astrological date range for Leo the Lion is July 23 - August 23. The astrological zodiac dates refer to the Sun's seasonal locations as they were about 2,000 years ago. Since that time, precession, the glacially slow but inexorable shift of the planet's celestial poles, has altered the Sun's position along the zodiac enough to have changed the dates by more than three weeks.
SATURDAY, SEPTEMBER 25: MOON NEAR THE PLEIADES
See the waning gibbous moon (81% illuminated) near the Pleiades, a star cluster in Taurus the Bull.
The Pleiades Star Cluster, located in Taurus the Bull, is approximately 440 light years away. The moon, by contrast, is 1.2 light seconds away. Although the moon is in the gibbous phase tonight, its light will not be enough to completely obscure this cluster.
SUNDAY, SEPTEMBER 26: MOON AT APOGEE
An :"academic event," meaning that there really is nothing to see. We include this event simply because it is of interest. When at apogee, the moon is at its farthest point from Earth during this particular orbit. The distance equals 404,640 kilometers, or 250,877 miles. If we could compare this gibbous moon to a gibbous moon during perigee, we would notice the size difference. However, as we cannot view the moon in side-by-side comparisons, the size reduction will go unnoticed.
TUESDAY, SEPTEMBER 28: LAST QUARTER MOON
PLANET WATCH
MERCURY: Planets often favor one hemisphere over another. Or, more precisely, the planet's angle relative to the ecliptic makes it easier for some observers to see than others. This month, Mercury is not in prime viewing location for us northern hemisphere dwellers. As we're also losing summer this month, consider Mercury's elusiveness to be the kind of double whammy that makes one wonder if the Universe is truly benevolent at heart. VERDICT: Best to see Mercury around mid-month when it is at greatest elongation. However, don't overexert yourself in your attempt to locate it as it will remain quite low along the western horizon.
VENUS: (PI PLANET!)
JUPITER (CK PLANET!)
Oh, we just could NOT decide which planet to designate the pick planet: Venus or Jupiter. They will both be brilliant and easily visible in the evening sky. While Venus will be sinking in the western sky, Jupiter will ascend in the east. Yes, Venus will be about three times brighter than Jupiter. However, Venus enjoys the unfair advantage of always being closer to Earth. Then again, Jupiter has an immense size advantage. (One could fit 1400 Venus-sized worlds inside the fifth world) We could deliberate this matter for hours. Instead, they are both beautiful evening sky beacons this month and so we'll confer the coveted "Pick Planet" designation on them both. VERDICT: Go out just after dusk and behold Venus in the west and Jupiter in the east!
MARS: Not visible this month.
SATURN: Jupiter's second fiddle. It is about as far from Jupiter as Jupiter is from the Sun and so has a distinct distance disadvantage. However, Saturn can be easily found by first finding Jupiter. Saturn will be the dimmer planet off to the northwest. A line connecting the two planets forms the hypotenuse of a thin right triangle. VERDICT: If you're out admiring Jupiter, anyway, you might as well seek our Saturn in the eastern evening sky. Although Saturn is seventeen times dimmer than Jupiter, it will still be conspicuously bright.
*The autumnal equinox will occur on September 21st in 2092 and 2096, just to be difficult.
To subscribe or unsubscribe from the Daily Astronomer: