THE SOUTHWORTH PLANETARIUM

207-780-4249   www.usm.maine.edu/planet

70 Falmouth Street   Portland, Maine 04103

43.6667° N                   70.2667° W 

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.

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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