THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 2458821.16
2019-2020: LXIV
"Something beautiful is languishing in the antechamber of your
imagination. It cannot speak to you directly, but only makes its presence
known as a lump in the throat or a pain in the stomach you feel when you
watch summer dissolve into autumn or see the dusk fires fading to black."
THE DAILY ASTRONOMER
Wednesday, December 4, 2019
Planet Light, Planet Bright
I: Inferiors
How can one distinguish between planets and stars in the sky? Sometimes
that discernment isn't as easy as one might think because planets resemble
stars, except they generally don't twinkle. Also, they tend to be brighter
than most stars. However, this statement isn't always true for every
planet. The concept of planetary brightness is the focus of
the latest subscriber question:
*"I understand why we can see stars; that are emitting their own light.
But why are the planets so bright? Is it just reflected sunlight? Even
reflecting off the gas giants? The planets seem a lot brighter than
moonshine, relatively speaking. " -S.H. Earth*
Excellent question!
First, the moon and the planets are all reflecting sunlight, hence their
visibility. Some of the planets are immensely bright all the time, such as
Venus. Other planets, such as Mars, are at times dim and at other times
dazzling. To answer the question about why planets are so bright, we
need to discuss the causes of planetary brightness and what factors are
involved in determining how bright a planet appears to us.
As this answer is a bit involved, we want to divide the response into two
parts. Today, the focus is on inferior planet brightness. Tomorrow, we
conclude with a discussion about superior planet brightness.
Before we proceed, we should spend a moment discussing "magnitude," the
system astronomers use to measure the brightness of celestial objects.
The brighter the object, the lower the magnitude value. For instance,
Sirius, the brightest night sky star, has a magnitude of -1.42. The
brightest planet Venus has a magnitude of -5.0 when at maximum brightness.
The dimmest stars visible to the unaided eye are magnitude 6.0 On
this logarithmic scale, a star of magnitude 1.0 is 2.5 times brighter than
a star of magnitude 2.0.
Inferior planets are closer to the Sun than Earth: Mercury and Venus.
The factors involved in inferior planet brightness are distance from the
Sun; distance from Earth; albedo (the ratio of reflected to received
sunlight) size and phase angle. The closer a planet revolves around the
Sun, the more intense the incidental light on it will be. As light
intensity diminishes with the square of the distance separating the light
source and observer, the planet's distance from Earth is also quite
important. If you double your distance from a light, its brightness will
be reduced to one quarter of its original value. If you triple the
distance, the brightness will be lowered to 1/9th of the brightness.
Albedo determines how much light the planet reflects into space as opposed
to that which it absorbs. The higher the reflection, the brighter it
will be. The planet size also contributes to the brightness. The larger
the planet, the greater the surface area from which it reflects that
light. Finally, for the inferior planets, phase angle is a factor. The
inferior planets go through a phase cycle, just like the moon. The greater
the phase angle, the more light is directed toward Earth.
[image: mt-mercury-configurations-icon.jpg]
Mercury is the closest planet to the Sun, but its distance from Earth
varies from a minimum of 77 million kilometers to a maximum of 222 million
kilometers.* (Of course, when Mercury is at its maximum distance, it
will be on the far side of the Sun and not visible anyway. When at its
minimum distance , it will be between the Sun and Earth and also not
visible, unless it is in transit.) While Mercury is closest to the Sun
and can sometimes draw relatively close to Earth, it doesn't reflect much
light. Its mean albedo is 0.06, meaning that it reflects about six percent
of the sunlight that strikes it. Also, the surface area of tiny
Mercury is only about 10% that of Earth. As it is so close to the Sun,
Mercury can only veer about 28 degrees away from the Sun in our sky at its
point of maximum elongation. When visible in the night sky, Mercury at
its brightest can shine at magnitude -1.9. However, its magnitude value is
generally about 0, making it about the 5th - 6th brightest object in the
sky,
Venus is the second closest planet to the Sun. Its distance from Earth can
range between 38 million kilometers to 261 million kilometers. (Again,
when Venus is at its maximum distance, it will be on the far side of the
Sun and not visible anyway. When at its minimum distance , it will be
between the Sun and Earth and also not visible, unless it is in transit.)
Although it is farther from the Sun than Mercury, it can draw much closer
to Earth. Venus also has the advantage of size and albedo. It is
almost as large as Earth and its surface area is 90% that of our planet.
Its mean albeo is 0.75, meaning that it reflects 75% of sunlight back to
Earth.** Being farther from the Sun than Mercury allows Venus to move
about 47 degrees from the Sun in our sky at greatest elongation. These
factors explain why Veus is the brightest planet. -3.9 at minimum to -5.0
at maximum,
Tomorrow, the brightness factors for the superior planets.
*Mercury's distance from the Sun varies considerably, as well, due to its
elongated orbit, but we've decided to just ignore this factor.
*A shout out to the nit pickers. Yes, this value depends on the wavelength
of the light and is averaged over the entire planet.
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