[image: f6b497e6a19e591d7a5efc8297010b2aa2d03a64r1-1920-1920v2_uhq.jpg]
*Iris: goddess of rainbows*
Let's imagine that a Greek god  or goddess who, after having been relegated
to the status of mythological figure since the 5th century AD, was so
desperate for worshippers that he/she approaches you and swears by the
River Styx to do anything you wanted him/her to do.   In response you say
something like,  "Apollo, stand on your head," or "Zeus, join the MeToo
movement," or "Athena, well, I don't know, smile or something."  Provided
that the desperate deity swore by the River Styx to fulfill your request,
you could be well assured that he/she would do so.      Were a god/goddess
to break such a vow, he/she would be cast into Tarturus for seven times
seventy years.  During that (let me fetch a calculator....) During that 490
year stay, the doomed deity would not only be deprived of all divine
powers, he/she would be enfeebled to the point of paralysis, tormented by
harpies, and rendered miserable beyond all description.    After his/her
confinement period ended, of course, he/she would be released back to
Olympus, his/her godhead would be restored and all would be well, except,
of course, when he/she realized that -ha ha ha- he's/she's still just a
mythological figure.     So, in this circumstance,  if you commanded Apollo
to stand on his head, then you'd soon be admiring his upturned feet.
The River Styx is the meandering river that originates from Mount Chelmus
in Arcadia and descends into the underworld where it wraps around Hades'
death kingdom nine times (the inspiration of Dante's nine circles.)   The
ferryman Charon conveys the spirits of the recently deceased across this
river.   When a god swears by the River Styx, Iris, the goddess of
rainbows, gathers some of its waters in a jar and pours it over the god's
hands, thereby sealing the compact.    Delivering this water is Iris's
secondary function in the mythological Universe.   Iris serves first and
foremost as a messenger.    Unlike the winged Hermes who communicates
principally between the gods, Iris conveys messages from Olympus to Earth
by descending along the rainbow arcs: the only conduit connecting them.
 Being both intangible and inaccessible, rainbows were the perfect bridge
along which Iris could travel.  No flesh-bound mortal was capable of even
reaching it, let alone standing or walking on it.    Being the daughter of
the ocean deity Thaumas and oceanid Electra, Iris is one of the older
deities and so rarely becomes embroiled in the fights waged between the
other deities.  She is, instead, a serene sort of goddess who often naps in
the shadow of Hera's throne until summoned to deliver a message by one of
the Olympians.     How appropriate that she chose Zephyrus, the west wind,
as her husband.  He is known for being the gentlest of all the winds, a
counterpoise to Boreas, the assailing north wind harsh enough to kill even
the heartiest mortal.       Western zephyrs, like afternoon rainbows, are
the insinuations of Olympian beauty into our mortal realm and as such
delight the senses as much as they enliven the spirit.    The rainbow
bridges that extend between the highest climes and lowest tiers still
appear from time to time and shall forever remain in our sights but well
beyond our reach.

THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 2459024.16
2019-2020:  CLXVII

THE DAILY ASTRONOMER
Tuesday, June 30, 2020
Remote Planetarium 55:   The "Invisible" Lunar Eclipse

Current circumstances compel us to deviate slightly from our itinerary.
 We progressed beyond the solar system's bounds around mid-May and are
preparing to  begin an exploration of variable stars.    Before we proceed,
however, we return to lunar eclipses.     The impending* "Independence Day
Penumbral Lunar Eclipse!!"   *necessitates this rapid revistiation of lunar
eclipses.   (Tomorrow we start the variable star sequence.)

As the catchy name implies, this eclipse occurs on July 4th.  Well,
technically, it begins on July 4th and concludes on July 5th.
 Fortunately, as indicated in the map below, observers in our region will
see the entire eclipse.

[image: Visibility_Lunar_Eclipse_2020-07-05.png]

Unfortunately, there is nothing  much to see. This upcoming event is
actually a "partial penumbral eclipse," meaning that the only a section of
the moon will move through the penumbra, the outer region of Earth's
shadow.     Those eager skywatchers hoping to behold a celestial spectacle
will instead experience a crashing disappointment.   Whereas a total
penumbral eclipse subtly darkens the entire moon, a partial penumbral
eclipse casts a very faint shadow only on one part of it.       Even at
"maximum" eclipse, that shadow is imperceptibly faint.

*So, why bother to devote an entire class to it?*
Well, in the first place, this eclipse has garnered unexpected media
coverage.  Many people are hoping to see a gorgeous lunar eclipse after
watching the fireworks display. Realize that municipal authorities might
put the kibosh on the pyrotechnics due to social distancing requirements
and the current condition of the landscape which is so parched a hostile
glance could inflame it.    It is our task to be wet blankets and tell you
that the follow up eclipse will prove to be a non-event, as well.

Secondly, this eclipse does give us another opportunity to discuss the *Saros
cycle.  *Every solar and lunar eclipse is part of Saros cycle of eclipses.
 Each eclipse within a given Saros shares a similar geometry and position
with all the others in the same cycle.   Each successive eclipse within the
Saros is separated by 18 years and 11 days.

This impending eclipse is the *3rd *of 71 lunar eclipses within Lunar Saros
Cycle 149.   What we will be witnessing (though not seeing) on July 4th/5th
is the beginning of a Saros cycle that started in 1984 and will finally end
in the year 3246.


The first Saros 149 eclipse occurred on June 13, 1984.  [In that same
month, Cirque du Soleil was founded; and, quite importantly, the final
episode of the Britcom "The Young Ones" was broadcast on BBC 2.]   This was
a *partial penumbral eclipse *during which only a small part of the moon's
upper regions passed through the penumbra.

The second Saros 149 event happened on June 24, 2002.  [Earlier that year,
the 2001 Mars Odyssey Space Probe began mapping the Martian surface.]
The image below shows the moon's path during that partial penumbral
eclipse.   Only a small section of the moon glided through the outer shadow.

[image: 480px-Lunar_eclipse_chart_close-2002Jun24.png]

The third 149 Saros cycle eclipse occurs on July 4/5th.   Notice the moon's
path during that event:

[image: 480px-Lunar_eclipse_chart_close-2020Jul05.png]
The moon's path is slightly higher than the one it followed in 2002.
 Consequently, more of the moon will pass through the penumbra.   Still,
the resultant darkening will be negligible.

When will the next Saros 149 lunar eclipse occur?

18 years and 11 days later:     *July 16, 2038*
What can we conclude about this eclipse?
First, we know the moon's path will be higher during this eclipse than it
was in 2020.  It will still be a partial penumbral eclipse, meaning that it
won't prove to be a dazzling event.

[image: Lunar_eclipse_chart_close-2038Jul16.png]
We can see that the moon's path is higher and more of it passes through the
penumbra.        Each Saros Cycle 149 eclipse happens at the moon's *descending
node,* the intersection point between Earth's orbit and the moon's.  Each
successive 149 eclipse moves the moon northward.
____________________________________
Remember that lunar eclipses that are part of even numbered Saros cycles
occur at the ascending note.   Lunar eclipses that are part of an odd
number Saros cycles occur at the descending node.
____________________________________

Can you predict the dates of the next three 149 Saros cycle eclipses?


   - *July 16, 2056*
   - *August 7, 2074*
   - *August 17, 2092*

All of those eclipses will still be penumbral.   However, the following 149
lunar eclipse on *August 29, 2110,* will be partial, albeit only just. A
thin sliver of the moon will pass through the* umbra,* the dark, inner
region of Earth's shadow.

The first total lunar eclipse in this Saros cycle will occur on *April 16,
2489*.  We see in the image below that the entire moon will move through
the umbra.

[image: LE2489-04-16T.gif]

 By studying the world map below we see that observers within our throat of
the forest will be enthralled by the eclipse which will be in progress at
moon rise.  Assuming, of course, that our descendants will not have evolved
to the level of demi-god and won't consider such trivial celestial events
worthy of notice.

Using our knowledge of Saros cycles, we can expect the following 149 lunar
eclipse to occur on *April 28, 2507*!     Yes, that total lunar eclipse
will be of longer duration because the moon's path through the umbra will
be higher.

The last total lunar eclipse in 149 will happen on *September 17, 2741.*
The last lunar eclipse -also a partial penumbral- occurs on *July 20, 3246.*

While the upcoming eclipse event won't scintillate the senses, it does
allow us the opportunity to contemplate the power of mathematical astronomy
to make accurate predictions about the remote future.

_____________________________
*Today's skywatching tip:*
Don't bother venturing out to observe the July 4th/5th penumbral lunar
eclipse.    Wait for the beautiful lunar eclipse on November 19, 2021.
Although technically partial, this eclipse will appear nearly total.
______________________________

Tomorrow, back out into deep space with *extrinsic variable stars.*



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