[image: jacob-jordaens-cadmos-and-minerva_orig.jpg]
*Spartoi: The "Sown Men"*
After Zeus, in the guise of a bull, spirited away the beautiful maiden
Europa, her father, the Phoenician king Agenor, commanded her brother
Cadmus to find her, even if he had to travel through the entire world. The
dutiful Cadmus and an entourage of servants sought her out for many months,
but to no avail. Frustrated, he consulted the Delphic Oracle and was
instructed to discontinue the search despite the enmity the abandonment
would engender in his father. "Europa is with Zeus and cannot be
retrieved," the Pythoness explained. "You must be the founder of a great
city. Find a cow with a crescent moon imprinted on its haunch. Follow
it until it lies down. At its location of repose, there shall you
establish your city." More fearful of incurring Apollo's displeasure
than that of Agenor, Cadmus and his entourage set out to find the cow,
which they soon located. The crescent moon mark made it rather
conspicuous. They tracked the cow for more than three days before it
laid down. Cadmus and his men stopped at that very spot to begin the
construction of the new city.
First, however, Cadmus knew the gods well enough to realize that he had to
sacrifice the cow back to them. He instructed some of his men to go to a
nearby spring and fetch the water needed for the sacrifice ritual.
Unfortunately for the men, a fierce dragon beloved of the war god Area
guarded the spring. The Dragon devoured them as soon as the men appeared
before it. After more than an hour passed since the men left, Cadmus
traveled to the spring and found the dragon dozing after its hearty meal.
Seizing his advantage, Cadmus withdrew his sword and attacked. The dragon
awakened just in time to see the intrepid Cadmus plunge the sword into its
throat. While Cadmus looked down onto the slain dragon, the goddess
Athena appeared to him. "Sow the dragon's teeth into the very ground
where you are building your city." Cadmus snapped out each of the
dragon's teeth and planted them all like seeds. To his amazement, a
fully armed man arose from each tooth. Frightened by the army that
appeared before him, Cadmus tossed a stone into the throng of soldiers.
Believing that the stone was thrown directly at him by one of the other
armed men, each soldier withdrew his sword and began to fight. After the
ensuing melee, only five armed men remained. Knowing that these
soldiers were the strongest of the lot, hence their survival, Cadmus made
them all co-founders of the famous city that became known as Thebes.
THE SOUTHWORTH PLANETARIUM
207-780-4249 www.usm.maine.edu/planet
<http://www.google.com/url?q=http%3A%2F%2Fwww.usm.maine.edu%2Fplanet&sa=D&sntz=1&usg=AFQjCNHulkHuLP13bOG2PkNrPazsGWFs2A>
70 Falmouth Street Portland, Maine 04103
43.6667° N 70.2667° W
Altitude: 10 feet below sea level
Founded January 1970
Julian Date: 2459265.18
2020-2021: XCV
"Sent on purpose."
THE DAILY ASTRONOMER
Monday, February 22, 2021
Exploratorium XXVIII: Lunar Eclipse Questions
*During a total lunar eclipse, would a moon-based observer see a solar
eclipse like we would see on Earth when the moon crosses in front of the
Sun? *
Observers on the moon's surface would not see the same type of solar
eclipse that we see on Earth when the moon moves directly in front of the
Sun. First of all, Earth does not appear to move across the moon's
sky. As the moon is tidally locked with Earth, our planet maintains the
same position in the moon's sky (apart from minor shifts related to
secondary motion effects.) Astronomers refer to this tidal lock as
synchronous rotation, meaning that the moon's rotational period is equal to
its revolutionary period. Consequently, the moon directs the same face
toward Earth as it revolves around it. Although the moon has no dark
side, Pink, it does have a "far side," a side which never faces Earth.
Observers on this far side would never see our home planet. Those on the
near side would see Earth occupy only one position.
[image: OSC_Astro_04_05_Rotation-1.jpg]
Synchronous rotation: the moon's rotational period is equal to its
revolutionary period. Any moon-based observer would either not see Earth
at all (if he/she were on the far side of the moon) or would only see the
Earth in one location in the moon's sky.
The second issue pertains to angular diameter. During a total solar
eclipse, the moon covers the Sun precisely because their angular diameters
are just about equal. Although the Sun's diameter is about 400 times
greater than the Moon's, it is also 400 times farther away. Both
angular diameters equal approximately one-half a degree. * When seen from
the moon, Earth's angular diameter ranges from 1.8 - 2 degrees, making it
appear significantly larger than the moon. So, during a total lunar eclipse
as seen on Earth, a moon-based observer would see the Sun appear to move
behind Earth. However, that observer would not witness a spectacle akin to
a total solar eclipse: Instead, the Sun would simply vanish behind Earth
before slowly emerging into view again.
[image: 788834b5c6f14e1ba3d942df3fc59810_18.jpeg]
During a total lunar eclipse, an observer on the moon would not see
exactly the same type of solar eclipse we Earthlings sometimes witness.
Earth appears much larger than the Sun from the perspective of the moon's
surface. The Sun would appear to simply vanish behind Earth.
*Why does the moon actually appear reddish during a total solar eclipse and
how does that relate to the Danjon Scale?*
Because the Sun is always shining on Earth, the planet always projects a
shadow cone into space. (We live in the base of that cone every night.)
During a lunar eclipse, the full moon encounters this shadow. During an
eclipse, the moon appears red because it is reflecting the red light which
is constantly directed into the cone. That light derives from the
sunsets and sunrises that occur around the planet's edge.
[image: LEDiagram1c.jpg]
The Danjon Scale, devised by French astronomer André-Louis Danjon
(1890-1967), measures the relative brightness and coloration of lunar
eclipses. Danjon established this scale to try to find a correlation
between the appearance of lunar eclipses and the solar cycle.**
The Danjon Scale has a range between 0 - 4:
- *0 * Very dark eclipse. Moon might even not be visible during
totality.
- *1 * Brown or grey coloration.
- *2 * Rust-colored eclipse. Also deep red. Distinct color
differential, with the central moon dark and the outer edge brighter and
more colorful.
- *3 *Brick red eclipse. Yellowish rim around umbral shadow
- *4 * Very bright eclipse. Copper red or orange eclipse.
Umbral shadow has a bluish edge
[image: Tom_Ruen_-_Lunar_eclipses_in_2003_(pd).jpg]
The above image shows two eclipses, one from May 2003 and the other from
November 2003. The May eclipse image would be a 2 on the Danjon scale;
the November eclipse would rate a 4.
Danjon values cannot be predicted. Instead, they are observed during the
moment of greatest eclipse. Atmospheric factors tend to affect this
rating more than any other consideration. For instance, the December 9,
1992 lunar eclipse, seen below, was quite dark due to the 1991 eruption of
Mount Pinatubo.
[image: dec92ec2.jpg]]
Particles scattered into Earth;s atmosphere by a volcanic eruption in 1991
caused the 1992 lunar eclipse to appear quite dark, as seen in the 4-minute
time exposure seen above. To the unaided eye, the reddish coloration we
see at the right side of this image would have been far less distinct.
During the next lunar eclipse on November 19, 2021, you should try to
determine the Danjon value for yourself during the maximum eclipse phase.
While that eclipse is technically partial, the reddish coloration will
still be apparent because most of the moon will pass into the umbra.
*Can any of the planets be eclipsed by Earth's shadow?*
The other planets will never seven stray close to Earth's shadow cone.
The inferior planets Mercury and Venus are much closer to the Sun than
Earth and wouldn't venture anywhere near the shadow cone, anyway. The
minimum possible separation distance between Earth and Mars can never be
less than 32 million miles: well past the shadow cone tip, which is about
840,000 miles from Earth.
*You mentioned that Saros Cycles have finite durations. Can astronomers
know when future Saros cycles will occur?*
Absolutely. Astronomers know about Saros that will begin centuries in the
future. For instance, Lunar Saros 180 will begin with a partial penbumbral
eclipse on August 28 , 2827 and will end with another partial penumbral
eclipse on October 3, 4089.
Lunar Saros 150 began on May 25, 2013 and will end on June 30, 3275.
Lunar Saros 151 will begin on June 6, 2096 and won't end until July 13,
3358.
*The moon is moving away from Earth. Will lunar eclipses ever cease?*Well,
lunar eclipses will eventually cease after the Sun ends its life cycle. Or,
the expanding red giant sun could incinerate Earth, putting an end to lunar
eclipses as well. Of course, those scenarios are billions of years in
the future. The moon is receding from Earth at the rate of 3.8
centimeters per year. This gradual, but inexorable recession will never
push the moon beyond Earth's shadow cone. We should note that this
recession rate is not constant and will be reduced dramatically once the
oceans vanish on Earth in about 1.2 billion years from now as a consequence
of Earth's rising temperature related to the increasing solar luminosity.
*When will the next lunar saros cycle come to an end?*
Lunar Saros 110 will end on July 18, 2027. This Saros cycle started with
a penumbral eclipse on May 28, 747 . The last eclipse in the series
will be penumbral as well, meaning that it will pass unnoticed by most.
The previous lunar Saros cycle (109) ended on August 8, 1998. The
following Saros Cycle (112) will end on July 19, 2092.
*Isn't there always a solar eclipse associated with a lunar eclipse? When
will that solar eclipse occur that is associated with the November 2021
eclipse and will we see it?*
Yes. A solar eclipse either follows or precedes a lunar eclipse.
The November 2021 lunar eclipse will be followed by a solar eclipse on
December 4, 2021. However, we here in New England will be far away from
the totality path, which slices across Antarctica.
*This value varies as the moon's distance from Earth and Earth's from the
Sun are not constant.
**Many astronomers believe that the solar cycle does exert some influence
in this coloration.