Jul 18 2015
When I host star parties, one of the most enjoyable experiences is to introduce people to Saturn for the first time – “Saturn Virgins” they are called. People oooh! and ahhh! and walk around to the front of the telescope to see if I’m fooling them. The can’t believe that the solar system appears with such 3 dimensional depth and reality.
This is inevitably followed up with “let’s look at the other planets.” Jupiter is pretty cool, and occasionally shows moon shadows moving across it. Mars and Venus can be very bright, but Neptune and Uranus are just small dots, barely discernable as disks instead of single dots (as stars look).
But Pluto is a different story. Besides its demotion from planet to minor planet (a topic which generates immense debate, but which I’m firmly an agnostic), it is really far away. It is only visible with light reflected from the sun. The light from sun diminishes according to the inverse square law. If a planet is 10 times as far from the sun as another, then it gets 1/100th the light. But that is just the light falling on Pluto. That light has to reflect and come back to Earth, which is another inverse square law relationship, which makes it an “inverse power of 4” law. Moving a planet twice as far away makes it 1/16th as bright. Pluto is very far away, as far as solar system metrics go. It takes light about 4.5 hrs to go from Pluto to earth. Kuiper Belt objects become very dim, very quickly.
I have a scale model of the solar system in my back yard. I shrunk the sun to the size of a golf ball. To scale, Earth is then 12 feet away. Pluto is 330 feet away. This is seriously Far Away with Not Much In Between. And the light we see from Pluto is Magnitude -14 – requiring a serious telescope to see. Pluto is about 1 million times dimmer than Saturn.
Just before the New Horizons encounter with Pluto, I took some time lapse images of Pluto moving across the sky. It was impossible for me to see the spacecraft, and even detecting Pluto was a challenge. I set used my backyard observatory, the Cosmos Research Center, to photograph the sky around Pluto. This is what I saw:
This image is about 1 degree wide, about as wide as your index finger held at arm’s length. For those of you who can’t see Pluto yet, here is a close up, showing a zoom area around Pluto:
And for those of you who are still missing Pluto, here is a closeup showing the motion of Pluto over 4.5 hours – the same time that it takes for New Horizons to send information back to Earth. Pluto’s motion is shown as a sequence of dots, making a thin line across the middle of the frame. This shows were Pluto was when New Horizons sends a message (on the left), and where it is when we receive it (on the right).
And here is an animated image, showing the motion of Pluto over 4.5 hours. Look in the center for the dot moving across the image. If New Horizons sent a message while at the left most point in the motion, Earth would see it at the right most point.
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