The Fates
What is your destiny? How long will you live and what travails will beset you during your life? The Moirai, or the "Fates" made this determination for every mortal. Sired by Nyx, the god of night, the Fates consisted of three sisters. Clotho (the "Spinner"), Lachesis (the "Alloter") and Atropos (the "Inflexible.) They wove each tapestry with various threads and then severed it at life's end. The more adventurous the life, the more variegated the colors. The longer the life span, the longer the tapestry. So powerful were the Fates that even Zeus, himself, had no power over them, apart from ensuring that neither he nor any other Olympian would have his/her destiny decided by any of the sisters. They were known at times to have assisted Zeus in his battles, most notably when the king god and his Olympian cohorts were fighting a fierce war against Typhon, their most formidable enemy. The fates persuaded Typhon to partake of food that weakened him enough to allow Zeus and the other gods to prevail in a struggle that threatened to destroy the entire Universe.
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: 245946.16
Julian Date: 245946.16
2019-2020: CXX
THE DAILY ASTRONOMER
Monday, April 6, 2020
Monday, April 6, 2020
Remote Planetarium 6: More Moon Mischief
Welcome to week 2 of the "Remote Planetarium," an daily on-line course designed to offer introductory astronomy instruction. We truly hope you're enjoying these sessions and that they prove educational. Last week we ended with a discussion about the moon.
Let's quickly review what we discussed about the moon last week.
-The Sun always illuminates half of the moon. We see phases because of the moon's changing orientation to the moon-Sun line.
NEW MOON (conjunction): when the moon is between the Sun and Earth. Illumination 0%
FIRST QUARTER MOON: when the moon is at a right angle relative to the Sun-Earth line. Illumination 50%
FULL MOON (Opposition) when the moon is on the far side of Earth relative to the Sun. Illumination 100%
FULL MOON (Opposition) when the moon is on the far side of Earth relative to the Sun. Illumination 100%
LAST QUARTER: when the moon is at a right angle again relative to the Sun-Earth line. Illumination: 50%
The waxing crescent phase separates the new moon and first quarter moon.
The waxing gibbous moon separates the first quarter and full moon.
The waning gibbous moon separates the full moon and last quarter moon
The waning crescent moon separates the last quarter and new moon.
The moon is always on or near the ecliptic because the moon's orbital inclination to Earth's orbit is slightly more than five degrees.
We now proceed to the next part of our lunar exploration. As we did last week, we'll provide a list of moon facts and from these facts derive other information about it.
4. THE MOON DOES ROTATE!
This is a tricky matter. One would think it doesn't rotate because we always see the same side of it no matter its phase. From this observation it seems natural to conclude that the moon doesn't rotate on its axis. However, it does. We only see one side of the moon because its orbital period equals its rotation period.
The moon requires about 27.5 days to complete one orbit around Earth, the same time the moon needs to complete one turn. This equivalence results from a phenomenon called "tidal locking," which often happens when one object orbits another.
Try this demonstration at home: draw a face on a tennis ball or softball or any other sphere. Next, choose a friend to be a parent body and orbit around him while turning the ball at the same rate. He or she will notice that the face is always directed toward him or her.
5. THE LUNAR CYCLE IS 29.5 DAYS LONG
In other words, the time span separating successive full moons is 29.5 days long. One might wonder: why is the lunar cycle 29.5 days long when the moon only requires 27.5 days to complete an orbit?
First, we know that the moon appears full when it is on the far side of the Sun relative to Earth. We call that phase "opposition." For a moment, we're going to suspend all the physical laws and pretend that Earth remains at the same point in its orbit. Were Earth stationary, then the moon would require 27.5 days to travel from one opposition to another. However, in the real Universe, Earth revolves around the Sun while the moon revolves around Earth.
Let's say that the moon reaches opposition. After 27.5 days, it is not yet on the far side of Earth relative to the Sun because Earth has moved forward in its orbit by about 27 degrees. The moon requires two extra days to yet again be at opposition.
6. THE MOON MOVES ABOUT HALF A DEGREE AN HOUR; ABOUT 12 DEGREES EVERY DAY
To phrase it another way, the moon moves a distance equal to its own angular diameter every hour. This speed is not constant due to its changing distance (see next section) and its orientation relative to the ecliptic. The moon's average speed is about 2,288 miles per hour (3,683 kilometers per hour). Even though all the planets, even Pluto, are moving much more rapidly, from our perspective, the moon is the sky's fastest mover due to its close proximity. Speaking of which...
7. THE MOON'S DISTANCE VARIES THROUGHOUT ITS ORBIT
The moon's orbit is elliptical, not circular. Were the moon's orbit shaped like a circle, the moon would maintain a constant distance from Earth. As it travels along an elliptical orbit, the moon's distance varies from a minimum, called "perigee," to a maximum, called "apogee." Whereas the average distance separating the centers of Earth and the moon equals 239,229 miles, the average perigee distance is 221,500 miles and the average apogee distance equals 252,700 miles.
Tomorrow we will discuss elliptical orbits in much greater detail.
8. A SUPERMOON OCCURS WHEN THE MOON IS FULL AT OR AROUND PERIGEE
The term "supermoon" has come into vogue in recent years. This term refers to the full moon that occurs when the moon is near its closest point to Earth. Supermoons are not common because the period between successive perigees, called an "anomalistic month," is approximately 27.55 days. The time period separating successive full moons, called a "synodic month," equals 29.5 days. Only on occasion will the two cycles allow perigees and full moon to happen around the same time.
We can see the size difference between a full moon at perigee (a "supermoon") and a full moon at apogee, sometimes called a "micromoon" by those who can never get enough nifty terms. A supermoon appears about 14% larger when at perigee than at apogee. It will also be about 30% brighter, as well.
Speaking of which.
9. THE FULL MOON IS NINE TIMES BRIGHTER THAN THE QUARTER MOON.
This statement doesn't seem to make sense at all! Geometrically, one would think that the full moon would be only twice as bright as the quarter moon. It is nine times brighter because the moon reflects most of the sunlight directly back along the incidental path. Look at the first image we included in today's post. We can see that when the moon is full, Earth is directly within that path. When the moon is at first or last quarter, Earth is parallel to that path and so receives far less moon light. You have likely noticed that the full moon provides ample illumination for a nighttime walk even in the forest.
Added note: The moon only reflects about 7% of the sunlight striking it. Technically, we say that the moon's mean albedo is 0.07. One must wonder how bright the full moon would appear were it to reflect even fifty percent of the sunlight it receives!
10. THE MOON'S SURFACE GRAVITY IS ABOUT 17% AS STRONG AS EARTH'S.
Strangely, some people believe the moon has no gravity. In fact, every massive object -including you- exerts some gravitational pull on everything else. The magnitude of that gravitational attraction is related to the amount of material the object contains. It is also related to the separation distance between the objects, an issue we'll address in greater detail when we discuss Newton's laws next week.
The suits the Apollo astronauts wore weighed about 500 pounds on Earth, but only about 80 pounds on the moon. For this reason they were able to easily move around the lunar surface.
We will find as we proceed through the course that we could devote many hours to each celestial destination we visit. However, we have much ground (or sky) to cover and so must limit our time at each stop.
Tomorrow we discover the elegance of orbits with Kepler's three planetary laws.
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An Insufficient Dedication
It seems theoretically impossible to ever be sufficiently grateful for a mother's love. Not only does she draw you into this world, but is unfailing, though not always successful , in her efforts to protect you from its many perils. My gratitude falls far short of what is owed, just as these few lines will serve as a woefully inadequate tribute to Sue L. Gleason.
My mother left the world on Friday evening after almost 94 years of having lived earnestly upon it. It was only after a debilitating stroke had robbed her of mobility and consciousness that she finally answered the ultimate summons. A dignified surrender for a noble spirit that refused to yield an inch to time without a struggle.
Sue learned the hard art of struggle as one of nine children on a Virginia farm during the Great Depression. (Two died in youth: one in a car accident at the age of eight, and her twin who perished at four days old due to a malformed digestive system.) A childhood of such privation and marred by tragedy would have hardened a lesser soul. In Sue, such an upbringing instilled a need to leaven the burdens of others, be it patients in the hospital where she for years served as a volunteer, her three beloved children or the countless others who found solace merely by being in her presence. When my father died 15 years ago, Mother shelved her own profound grief to minister to those trying to cope with their own bereavement. In any crisis, it was her natural instinct to sustain others.
Above all, Sue Gleason taught us how genuine strength is so often misunderstood. True strength isn't aggression or outrage, but instead is the quiet determination to remain calmly benevolent in a turbulent world that so often punishes kindness. She understood that to maintain one's nature through time is to stand firm against hurricanes.
Mother, I know you would be most displeased by this section. You shied away from all spotlights and would have been content with a single line. A single line would not have been a sufficient dedication. Then again, 1000 pages would not have sufficed, either. I know that though you dearly loved this good Earth, such was the condition of your perpetually moving body that you probably weren't sorry when the summoning angel clasped your weakened hands and told you it was time to move on. As promised, we will scatter your ashes in the same remote river where your husband's ashes dissolved a decade and a half ago. You are far more suited for swift currents than encapsulating urns. I don't know to which receiving Heaven you are bound, but wherever it is the community that awaits you will be all the better off for having you as a part of it.
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