THE SOUTHWORTH PLANETARIUM
70 Falmouth Street Portland,Maine 04103
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Altitude: 10 feet below sea level
Founded January 1970
2021-2022: XXIV
“Once you can accept the universe as matter expanding into nothing that is something, wearing stripes with plaid comes easy.” -Albert Einstein
THE DAILY ASTRONOMER
Tuesday, October 5, 2021
October 2021 Night Sky Calendar Part II
WEDNESDAY, OCTOBER 6, 2021: UNSEEN SUPER MOON
We are accustomed to visible "super moons," defined as full moons that occur at or around perigee, the time when the moon is closest to Earth. The super full moon is 14% larger and 30% brighter than a "micro-moon," one which occurs around apogee, the point of greatest distance. Hence, the hype. However, one can also experience a super moon when the moon is new or at conjunction. Although the new moon isn't visible (eclipses apart), the unseen super moon still affects the tides. Tides occur due to differential gravity, or the way that a massive body exerts its gravitational influence on another body unevenly. For instance, the section of Earth closest to the moon experiences a stronger gravitational pull than other areas. The consequent pressure differential along the planet causes the tidal ebb and flow.
The moon's influence is responsible for the tides, but, so too, is the Sun, albeit to a lesser extent (44% as influential as the moon). When the Sun and moon are aligned, during conjunction or opposition (full moon), tides are higher. We refer to those higher tides as spring tides. High tides that occur during a supermoon are known as king tides, because they tend to be even higher than spring tides. To draw this issue out to its logical extreme, tides can be even higher than king tides. If we experience a super moon around the time when Earth is at perihelion, or its closest point to the Sun (sometime around January 1 - 5), we'll experience Poseidon's tides. The tides are profoundly affected by the moon and Sun's distance because differential gravity diminishes with the cube of the distance, not just the square.*
The highest possible astronomical tides are known as Poseidon's tides, which can only happen when a supermoon occurs at or around the time of perihelion. Super
The moon is new today and will reach perihelion on October 8th. A super moon!
However, the only people who will observe this supermoon are those who study tide tables. The rest of us will notice nothing at all.
FRIDAY, OCTOBER 8:MOON AT PERIGEE
The point of least separation distance between Earth and the moon. When at the actual perigee position (1:28 p.m. EDT), the moon will be 363,386 kilometers from Earth. See the previous passage for more information pertaining to the supermoon.
SATURDAY, OCTOBER 9: MERCURY AT INFERIOR SOLAR CONJUNCTION
Yet, again, another unseen celestial event. Today, Mercury will move between the Sun and Earth and so won't be visible. Unlike superior planets, those that are farther from the Sun than Earth, inferior planets can move between the Sun and Earth (inferior conjunction) or on the far side of the Sun relative to Earth (superior conjunction). Superior planets can never be between Earth and the Sun.
We won't experience a transit of Mercury today because transits can only occur when an inferior planet is at or near a node, an intersection point between its orbit and Earth's. Mercury will not be close enough to a node today. In fact, the next Mercurian transit will not occur until November 13, 2032! However, we'll have to wait until December 10-11, 2117 for the next transit of Venus!
SUNDAY, OCTOBER 10: MOON NEAR VENUS (BRONZE EVENT!)
Find the waxing crescent moon (22% illuminated) near Venus, the night sky's brightest planet. One will find few celestial sights as enchanting as the gathering of Luna and Aphrodite against twilight.
Venus will set by 7:47 p.m
The moon follows about an hour later.
WEDNESDAY, OCTOBER 13: FIRST QUARTER MOON
THURSDAY, OCTOBER 14: MOON NEAR SATURN
One can find Venus and Jupiter easily enough even if the moon doesn't appear close to them. Saturn, being 16 times dimmer than Jupiter, is not as readily observable. Tonight, sky watchers will see the waxing gibbous moon (66% illuminated) passing south of Saturn.
FRIDAY, OCTOBER 15, 2021: MOON NEAR JUPITER (SILVER EVENT!!)
Yesterday, we saw the moon near Saturn. Tonight, it appears close to Jupiter. The pairing of Jupiter and the moon is almost as beguiling as that of the moon and Venus. Of course, Jupiter and the moon will remain above the horizon until a couple of hours after midnight.
*What's the difference?
Well, if you double your distance from a massive object, its gravitational influence on you is reduced to one-fourth of its original value, or one divided by 2-squared (2 x 2). If you double your distance from the same massive object, the differential gravitational force is reduced to one-eighth its original value, or 2-cubed (2 x 2 x 2).
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