THE SOUTHWORTH PLANETARIUM
70 Falmouth Street      Portland, Maine 04103
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43.6667° N    70.2667° W  Founded January 1970
2022-2023: LXVII
Sunrise: 6:08 a.m.
Sunset: 5:37 p.m.
Civil twilight begins: 5:39 a.m.
Civil twilight ends: 6:06 p.m.
Sun's host constellation: Aquarius
Moon phase: *FULL MOON* (100% illuminated)
Moonrise: 5:51 p.m.
Moonset: 6:49 a.m. (03/08/23)
Julian date: 2460011.29
"Common sense is the collection of prejudices collected by the age of
eighteen."
-Albert Einstein

THE DAILY ASTRONOMER
Tuesday, March 7, 2023
Inside Earth

Consult any of the credible -and  most of the illegitimate- dictionaries
and you will find the following definition for astronomy:

              *astronomy (n): *the branch  of science that deals with
phenomena beyond Earth's atmosphere.    [It is regrettable that 'astronomy'
does not appear in Ambrose Bierce's 'Devil's Dictionary.]

Despite this conventional definition, today's DA looks down and through the
ground  to answer an intriguing question related to our knowledge of
Earth's complex interior.    This question was prompted by recent news that
Earth's inner core has either stopped rotating or has reversed its rotation
direction altogether.

*"I don't understand.  We haven't even gotten to the bottom of the ocean,
so how can we possibly know that Earth has a core or even  what's inside
our  planet?" M.B.*

Even though three people* have managed to travel to the Marianas Trench,
the lowest point within the ocean, you ask a very good question. How can we
possibly determine the sizes, locations and compositions of the zones
within Earth’s interior despite not being able to deploy any probes through
it? It is all done with seismic waves generated by Earthquakes.

We can separate these waves into two types.

   - P waves, or primary/pressure waves
   - S waves, or secondary waves

P waves are longitudinal compression waves, similar to the motion of a
slinky.

[Image credit: Narissa Spies]

S waves exhibit a motion perpendicular to the direction of the motion, like
that of a rope

[Image credit: Wikimedia Commons]

Whereas P waves travel along the direction of motion, S waves oscillate up
and down along the motion path:

[Image credit; Meghan Environmental Science]

Here’s the rub: P waves can travel through solids, liquids and even gases,
but S waves can only move through solids. Moreover, seismic waves travel at
different velocities through different types of materials.**

When an Earthquake -or another powerful wave-generating event such as a
volcanic eruption- occurs, seismic waves propagate through Earth in all
directions and can be detected by seismographs at other locations. By
studying the seismographs throughout these locations, scientists can
determine which waves are transmitted and the speed at which these waves
travel. Since S waves cannot travel through liquid, they won’t be detected
along any point within the ‘S-wave shadow zone, which extends along a
154-degree arc opposite the location of the seismic wave source (i.e.
Earthquake) However, P waves are detected at all locations except for two
more narrow “P shadow zone” regions. The refraction within the liquid outer
core leaves gaps that produce these two shadow zones.

[Image credits: US Geological Survey]

Since Earthquakes are quite lamentably common, scientists have been able to
quite accurately map the planet’s interior- the crust, mantle, liquid outer
core and solid inner core- by analyzing both of these wave types generated
by sources at multiple locations.

I do hope that this explanation, though simplified, proves helpful.

*Jacques Picard and Don Walsh (1960); James Cameron (yes, THAT James
Cameron) (2012)

*Sound waves also travel at different speeds through different media. For
instance, sound travels at about 767 miles per hour through air at room
temperature, but can travel at 10,200 miles per hour through copper. (Sound
travels very quickly through metals. Consequently, one can hear a train
approaching from quite a long distance merely by pressing an ear to a
railroad track.)




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