THE SOUTHWORTH PLANETARIUM
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Altitude:  10 feet below sea level
Founded January 1970
Julian Date: 2458821.16
2019-2020:  LXV
            "One doesn't truly know how long a light year is until the
entire driveway is shoveled."


THE DAILY ASTRONOMER
Thursday, December 5, 2019
Planet Light, Planet Bright
II:  Superiors

Yesterday we started the discussion about planetary brightness.    A
subscriber's question pertaining to the reasons planets seem to shine so
brightly prompted this discussion.   Yesterday we focused on the inferior
planets, those closer to the Sun than Earth. Today, we'll focus on the
brightness of the superior planets, those farther from the Sun than Earth.
These planets are, in order, Mars, Jupiter, Saturn, Uranus, Neptune and
Pluto, thank you very much.

Many factors determine a planet's brightness:   its distance from the Sun,
its distance from Earth, its size and its albedo, or the ratio of reflected
to received light.   (We have another complication with Saturn, which we'll
address later.)       These factors are all important because planets shine
by reflected sunlight.   Light intensity diminishes with the square of the
distance separating the light source and the observer.    Double your
distance from a light source and its intensity is reduced to one quarter of
its original value.    Triple your distance, and it is reduced to one ninth
of the initial intensity.      Albedo determines how much light the planet
reflects back out into space.  The size relates to the surface area from
which the light is reflected.

Superior planets will ALWAYS be closest to Earth at or around the time of
opposition, when Earth is passing between the planet and the Sun.
For this reason, the superior planet will also be at its brightest at this
time, as well.    We are going to focus only on the three closest
superiors, Mars, Jupiter and Saturn, as those planets are the only ones
visible to the unaided eye.*

*MARS*
Average distance from the Sun: 141 million kilometers
Distance from Earth at opposition:  34 - 63 million miles
Albedo:  0.15
Magnitude range: Maximum:  -2.9     Minimum : 1.9

Sometimes Mars dazzles and delights. At other times, it glows dimly and
shyly out of sight.  The red planet absorbs most of the meager sunlight it
receives, but does have the advantage of drawing closer to Earth and the
Sun than any of the other planets.

*JUPITER*
Average distance from the Sun:   779 million kilometers
Distance from Earth at opposition:  around 641 million kilometers
Albedo:  0.52
Magnitude range:  Maximum:  -2.9    Minimum:  -1.6

Yes, Jupiter is much farther from the Sun than Mars and never comes nearly
as close to Earth as the fourth planet.  However, Jupiter is enormous with
a surface area 122 times greater than that of our home world,  Moreover, it
is reflecting slightly more than half of all its light back out into space.
  Consequently, Jupiter can often be brighter than Mars.   Both Mars and
Jupiter are equally brilliant at maximum.

*SATURN*
Average distance from the Sun:  1.4 billion kilometers
Distance from Earth at opposition:  1.2 billion kilometers
Albedo: 0.35
Magnitude range:   Maximum:  -0.55   Minimum:  1.2

Even at maximum brightness, Saturn will never be as bright as Sirius, let
alone Jupiter or Venus even when they're at minimum brightness.    We must
remember that the outer planet separation distances are greater than the
separation distances of the inner planets.    Saturn is about as far from
Jupiter as Jupiter is from the Sun!      Also, it's outer gas layers are
absorbing most of the incidental sunlight.   Saturn reflects about 35% of
light back into space.    So, even at its brightest, Saturn shines only at
magnitude -0.55, slightly brighter than Alpha Centauri.

[image: 49781main_saturn1.jpg]
Saturn's various ring aspects:
the tilt of the rings changes over
time as the planet's orientation shifts
while it travels along its nearly 30-year long
orbital path.


Remarkably, though, Saturn wouldn't even be as bright as it appears without
the rings!     That complex ring system could add as much as a full
magnitude of brightness to the planet, depending on its aspect: the tilt of
the rings relative to Earth.       Although Saturn rarely ever dazzles
-except in a telescope- it would be even dimmer were it not for its
magnificent ring system.

So, S.H, you are right. Some planets shine brightly: even more so than the
moon, which, despite its close proximity to Earth, reflects only about 7%
of all the sunlight out into space.     The brightness varies because the
distance separating the planets from the Sun and Earth are constantly
changing: one of the innumerable reasons the night sky has proven so
captivating.



*No, that wasn't the sound of your furnace exploding.   It was the sound of
that person who just pounded the table and shouted, "I've seen Uranus with
my unaided eye, mister!"     To this retort we can only say,  "Dare you to
shout that in a middle school classroom."    Yes, Uranus is technically
within naked eye visibility, but just barely.   One would only find it in a
completely dark sky and the observer would have to be so seasoned they
could find make their way from Sirius to Ross 248 dizzy and blindfolded.


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