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Date:  2459316.18
2020-2021: CX



THE DAILY ASTRONOMER
Monday, April 12, 2021
Exploratorium XXXVIII:  Your Questions

A bit of time has passed since we've answered questions.  So, today,
as the article title indicates, we will answer some recent subscriber
questions.     Today's questions pertain to the Sun, moon, Arecibo
Radio Telescope and the effect of cosmic expansion on the human body.

*Since the Sun is much more massive than the moon, why is its
influence over Earth's tides just 44% that of the moon?  -F. Smith*
To answer this question, we must distinguish between gravity and
differential gravity.   The magnitude of the gravitational force two
massive objects exert on each other depends on the masses of both and
on their separation distance.   The greater the mass, the stronger the
force.  Conversely, the force diminishes rapidly -with the square of
the distance- as the bodies move away from each other.

                   [image: main-qimg-19af82f81b491283104a7b33a1e8e7cc.jpg]
The above idealized gravitational model illustrates the gravitational
force exerted between two "point masses," defined as massive objects
without any physical extent: points in space.     In the real world,
massive objects encompass a certain region of space.   Consequently,
the magnitude of the gravitational force exerted on a given object by
another will vary along the object.  The area on a body closer to the
other massive object experiences a greater force than the part farther
away.    This variation of gravity across a massive body is known as
*differential gravity*.

Because distance is an important factor in differential gravity, a
nearby massive object tends to exert a greater differential
gravitational force than a more distant object, even if the later
object is far more massive.     The moon and, to a lesser extent, the
Sun, are both responsible for Earth's tides.       Even though the Sun
is about 27 million times more massive than the moon, the moon is 400
times closer to Earth than the Sun and so has a greater effect on the
tides.

         [image: figure_showing_positions_sun_1473264128834091545_744.png]

Because distance is so important, the tides will be higher when the
moon is at or near perigee (its closest distance) than they are when
the moon is at or near apogee, its greatest distance.    *In summary,
although the Sun is far more massive than the moon, its fart greater
distance makes its contribution to the tides less than that of the
moon. *

*Can we see the point where the Big Bang occurred?*
*-Donald B.*
Yes and no.
First, look in front of you.   Now, look behind you.  Next, look down
the street.   Finally, look through a telescope toward, let's say, the
Andromeda Galaxy.    The Big Bang happened at all those points.   What
you're asking, though, is can astronomers pinpoint a specific point in
space in which the Big Bang happened?    The answer to that question
is no, because such a point doesn't exist.     According to the
prevailing cosmological theory, the Universe formed out of a singular
event dubbed *the Big Bang.*    Everything was created in that event,
including energy, matter, space and time.   A complete nothing.  In
fact, one cannot even say that "nothing existed before the Big Bang"
because the word "before" is temporally relative.

The Big Bang region is everywhere and also nowhere.

*Because the moon is moving away from Earth, will solar and lunar
eclipses ever stop?*
We will address this question in two parts:

*SOLAR ECLIPSES*
The moon is moving away from Earth at a speed of about 4 cm per year,
the same rate at which fingernails grow.   Although the moon's
distance from Earth won't increase substantially over a human
lifetime, the cumulative effect over millions of years will be
substantial..
In approximately 600 million years, the moon's distance will have
increased enough to make it always appear smaller in the sky than the
Sun.   Consequently, no more total solar eclipses will occur.
Instead, Earth will experience only annular solar eclipses: those in
which the moon doesn't entirely block the Sun so that a "ring of
light" is visible around the darkened moon.

           [image: 34ff8086-4dc4-4eeb-a4a3-aff7bd6de9a7.jpg]
Total solar eclipses will stop in about 600 million years. After that
time, the moon will appear smaller in the sky than the Sun.
Consequently, when the moon crosses directly in front of the Sun, an
annular eclipse will occur.


*LUNAR ECLIPSES*
Lunar eclipses occur when the full moon moves into Earth's shadow
cone.    This cone extends for approximately 840.000 miles.     By the
time the Sun's life cycle ends in more than five billion years, the
moon will not have moved even close to the end of this cone: less than
half the distances, actually.     Lunar eclipses will continue to
happen throughout the remainder of Earth's life.

*The Universe is expanding all the time. Does that mean that I am
expanding, too, and not just in the conventional sense?  *
*-S.S.*
The Universal expansion causes distant galaxies to recede from the
Milky Way.  Or, more correctly, to appear to recede from the Milky
Way.  In fact, all the galaxies are moving away from each other as a
consequence of the expansion.        However, this expansion is not
observable at smaller scales, such as at the level of the human being,
because local forces are far stronger.     The electromagnetic forces
within the body easily overpower such expansion.        Even the
gravitational attraction between close galaxies, such as the Milky Way
and Andromeda, is stronger than the Universal expansion.
Consequently, these galaxies are moving toward each other at 300,000
miles per hour and will collide in 4 - 6 billion years.

*I heard that the Arecibo Radio Telescope collapsed.  Will that affect
the Arecibo Radio message to the M13 Globular Cluster?   -C. Davis*
We should  first provide some background.
The Arecibo Radio Telescope, located in Puerto Rico, was the largest
radio telescope in the world.  However, toward the end of 2020, the
receiver platform collapsed into the dish, rendering it inoperable.

[image: 5fb6af3732f2170011f70178.jpg]

In 1974, astronomers beamed a "message" toward M13, a globular cluster
in the constellation Hercules.     Consisting of 1,679 bytes of data,
this message contains information about humanity, the solar system,
our number system, our basic chemical constituents, and even the radio
telescope that sent the message.     The astronomers used 1,679 bytes
because 1,679 is a *semi-prime number, *meaning that iit is the
product of two prime numbers, in this case 23 and 73.   When those
bytes are arranged in 23 columns and 73 rows, the following image
appears:

[image: 320px-Arecibo_message.svg.png]

The message is en route to the M13 Globular cluster, which is
approximately 25,000 light years away.    At this moment, the beam
continues toward its destination.  Despite the passage of almost fifty
years since the transmission, it has only traveled 0.18% of the entire
distance.    Although the Arecibo Radio Dish is now defunct, the
message it sent will continue to move toward M13.    The telescope's
destruction will not affect this transmission at all.



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