Saturday: At 5:30 a.m. PDT, the center of the Sun crosses the celestial equator and passes into the southern sky. The celestial equator is an imaginary line that divides the sky into a northern and southern half. When the Sun is in the southern half of the sky, it takes a shorter path from rising to setting. It also does not get as high in the sky at noon. This leads to shorter days and longer nights. Since the Sun crosses the celestial equator today, there is an instant when it is equally in the northern and southern sky, called the north and south celestial hemispheres. This so-called “equal night” is given by the Latin word equinox. Thus, today is known as the Autumnal Equinox. However, the day and night are not of equal duration today. The sun rises at 6:48 a.m. and sets at 7:00 p.m. in the northern latitudes of the United States. At these latitudes, day and night are closest to equal duration on Tuesday.
Sunday: “There’s water in them thar craters,” frozen water,
that is. There has been speculation since the 1960s and indirect evidence since
the 2000s of water on the Moon. Three years ago, astronomers studied data from
four earlier missions. They noticed that the light reflecting off the bottom of
craters near the lunar South Pole showed characteristics of light reflecting
off pure ice. The water likely came from comet impacts or other solar system
objects with trace amounts of water ice. Last summer, the Indian Space Research
Organization became the fourth country to successfully land on the moon when
Chandrayaan-3 landed near the lunar South Pole to study the water there. Read
more about it at https://www.space.com/chandrayaan-3-moon-south-pole-why-nasa-wants-to-go-too.
The moon is one fist above the north-northeast horizon at 11:00 p.m.
Monday: To the people of Ancient Greece, the stars that are
about five and a half fists above the northeastern and eastern horizons,
respectively, at 10:00 p.m. were known as Cassiopeia and Andromeda, a
mythological queen, and her daughter. But not all cultures imagined the same
pictures in the sky. To the people of Polynesia, the stars of Cassiopeia and
Andromeda represented a dolphin, called Kwu. Cassiopeia formed its tail, the
brightest stars of Andromeda formed its fins, and its fainter stars outlined
the dolphin’s body.
Tuesday: Ask someone which day in September has the same
duration day and night. Go ahead, ask someone. Why are you still reading this?
I can wait. If that person says the first day of autumn, they are wrong. Today,
three days after the first day of autumn, is the date in which day and night
are closest in duration. There are two main reasons for this. First, the
atmosphere acts like a lens, bending light from the Sun above the horizon when
the Sun is actually below the horizon. This makes the Sun appear to rise before
it actually rises and appear to set after it actually sets. Second, fall starts
when the center of the Sun passes through the point called the autumnal
equinox. But the Sun is not a point. The upper edge of the Sun rises about a
minute before the center of the Sun and the lower edge sets a minute after the
center of the Sun. Thus, even if we didn’t have an atmosphere that bends the
sunlight, daytime on the first day of autumn would still be longer than 12
hours.
Wednesday: Mars is less than a half a fist to the lower
right of the last quarter moon at 6:00 a.m. Jupiter is to the upper right of
the moon.
Thursday: Mnemonics are helpful for remembering astronomy
facts. (Similarly, “Johnny Mnemonic,” the 1995 cyberpunk film, was helpful in
getting Keanu Reeves’ career going.) After all, school children around the
world are learning the order of the planets by remembering, “My very excellent
mother just served us nine….” Oops, I guess that one needs updating with Pluto
being classified as a dwarf planet. Well, here’s one that will not need
updating for tens of thousands of years: the order of the stars in the Big
Dipper. Because the nighttime stars are so far away from us, their actual
motion through the sky, called their “proper motion” is not noticeable over
even thousands of years. That is why the constellations have remained the same
since ancient times. But two stars in the Big Dipper have a proper motion large
enough such that in 100,000 years; the stars will no longer make a dipper
shape. You can see this simulation at the American Museum of Natural History
video found at https://youtu.be/sBfUBtdo8yo.
Until then, you can remember the names of the seven dipper stars in order from
handle to cup by remembering this helpful advice for teens: “AM, ask mom. PM,
dad.” The stars are Alkaid, Mizar, Alioth, Megrez, Phad, Merek, and Duhbe. The
Big Dipper is one fist above the northern horizon at midnight.
Friday: Saturn is exactly two and a half fists above due
southeast at 9:00 p.m.
The positional information in this column about stars and
planets is typically accurate for the entire week. For up-to-date information
about the night sky, go to https://nightsky.jpl.nasa.gov/planner.cfm.
All times are Pacific Time unless noted.
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