Here are the answers to selected questions submitted between
. Questions may have been edited for clarity or brevity. Click on a link to move directly to the answer.
- I would like
to photograph star trails. How can I take these pictures?
What you need:
You absolutely need a tripod. The heavier the tripod, the better. If
your tripod is lightweight, try hanging stuff like your camera bag from
the supports, to make it heavier.
- You will need a camera that is reasonably "manual," as a one-button
completely automatic auto-everything camera will fail in this situation.
Ideally you want to be able to manually select the focus, the aperture,
and this next part is critical: You want to be able to take very long
exposures. On the kind of cameras I have in mind, there should be an
exposure setting called "B" or "bulb" --- this keeps the shutter open for
as long as the shutter release is pressed. So you can take exposures
that are as long as you want.
- You will need color negative film (the kind most people use), not slide
film (which is more common among professionals). The reason for this has
to do with contrast --- on color negative film you will be able to see
both faint and bright star trails, while on slide film you may only get
the bright ones. Film speed is not too crucial. Use 100, 200, 400
speed film, whatever you have around.
- You should use a cable-release or some other kind of remote control for
your camera's shutter. This will allow you to lock the shutter open
without actually holding your finger on the button for hours at a time.
Details on how this works will vary from one camera to the next.
- A wide-angle lens (small focal length: 28mm, for instance) will allow
you to photograph longer star trails and/or a bigger portion of the sky;
a narrow-angle lens (large focal length: 200mm, for instance) will show a
smaller portion of the sky.
What to do:
- Wait for a clear night with little or no moon. If you have to shoot in
the moonlight, at least point to a place that the moon won't come too near
(like the North Star, for instance).
- Set up your tripod in a reasonably calm place. If it's very windy the
camera will shake --- you don't want that.
- MAKE SURE your camera has fresh batteries. During these long exposures
you may run the battery down. See below if that happens.
- Set the focus at "infinity". If the camera has an auto-focus option,
you will probably have to turn this off and focus manually, because the
camera will have a hard time focusing on the blank sky.
- Set the aperture fairly wide open. This means a low "f" number.
Something like f/2.8, f/4, or f/5.6 should be available on your camera.
- If you want to see long, fairly straight star trails, point the camera
sort of overhead. If you would rather see circular trails, which show how
the earth rotates around a fixed axis that points toward Polaris (North
Star), then point the camera towards Polaris (due north).
- Open the shutter and lock it open with the cable release or remote
control. (On my camera, there is a setting where one press of the remote
button opens the shutter and another closes it, so this is pretty easy.
Details for your camera will vary.)
- Go to sleep, listen to music, whatever. Remember that it would take an
exposure of 24 hours in order to see the circular star trails around
Polaris go all the way around. You can't do that (because the Sun will
eventually come up), so experiment --- take exposures of an hour, a few
hours, etc, to get different length star trails. If you are using a
fairly narrow-angle lens, then really long star trails will run off the
edge of the picture. But if you are looking near Polaris where the trails
are circular you can still take really long exposures.
- When your exposure is done, release the shutter. BUT, YOUR BATTERIES
MAY HAVE DIED DURING THE EXPOSURE. Don't worry --- your film was still
exposing. Here's what you do: put the lens cap on the lens --- this
effectively ends the exposure. Next put fresh batteries in the camera,
and release (close) the shutter. Everything should be fine.
- Experiment and enjoy!
For more pages on astrophotography, see: Google's
(Thanks to Scott Seagroves for this answer!)
- Is Jupiter's Great
Red Spot a thunderstorm? If yes, how is it different of similar to
the ones on Earth?
The Great Red Spot on Jupiter is one of the most beautiful phenomena in
the solar system
Great Red Spot image).
The spot was first observed by the Italian-French astronomer
Jean-Dominique Cassini in 1655, roughly 350 years ago! Its size is 12,000
by 25,000 kilometers, big enough to hold two Earth's.
The Great Red Spot is indeed thought to be a giant hurricane-like storm
system. More exactly it would be classified as an anti-cyclone. Cyclones
on Earth are low pressure systems with stormy winds spiraling around the
calm center. The Coriolis effect, which is due Earth's rotation, causes
cyclones in the northern hemisphere to rotate counter-clockwise, and
clockwise in the southern hemisphere. The Great Red Spot, in contrast, is
a high pressure system, and is thus called an anti-cyclone. The
orientation of the rotation is flipped, and since the Great Red Spot is
located on the southern hemisphere of Jupiter it rotates
counter-clockwise. Its outer edge completes a rotation about every 6
days, which is much longer than the 10 hours that it takes Jupiter to
rotate about its own axis.
Incidentally, the Great Red Spot is currently in the process of colliding
with another great storm on Jupiter. The collision should last for about
a month. Most likely the Great Red Spot will emerge unharmed from this
scuffle, but the smaller one will most likely be disrupted or even
absorbed. The last time another storm collided with the Great Red Spot
was in 1975, and it caused the Great Red Spot to fade in color for
- Do we have rock samples from Mars here on Earth ? How?
Although we have sent many spacecraft on a one-way journey to Mars, no
missions have made the return trip to bring Mars rocks back to Earth.
Yet we have many rock samples that are fairly certain to be from Mars.
How can this be? All of these Mars rocks are meteorites- objects from
space which enter the Earth's atmosphere and have fallen to the ground
(while they are entering the atmosphere they glow brightly and are
often call falling stars). By studying what meteorites are made of,
we know that most are pieces of asteroids or comets. Once in a while,
however, the composition of a meteorite is exactly the same as robotic
surveys have found on Mars. This doesn't happen very often- of the
22,000 meteorite discovered on Earth so far, only 24 have been
identified as originating from Mars. We think that these Martian
rocks have been somehow kick off Mars, maybe during a large impact,
have wandered around the solar system and eventually found their way
to Earth. Since we have not yet been able to get rock samples
directly from Mars, these meteorites are very important for studying
the composition of Mars.
For pictures and more information about Martian meteorites, see JPL's Mars Meteorites page.
- What is an
An accretion disk is simply a disk of material that forms as the material
falls onto some massive object. The object in the center is gobbling up
(accreting) the disk material. People typically talk about accretion
disks around very massive and extremely compact objects, such as black
holes or neutron stars.
There are several artists conceptions of accretion disks around black holes:
conception of a disk around a spinning black hole.
conception of a disk in a binary star system.
- Here is a
real reconstructed image of an accretion disk.
An impressive animation of an artist's conception of the
accretion disk around an extremely large (hundreds of millions of times
the mass of the sun) black hole in the center of a galaxy. At the
beginning of the animation, you'll see a thin line pointing toward the
upper left hand corner. That's a jet, not the accretion disk
itself. Later in the animation you'll see a yellow disk with a black spot
in the center. The yellow disk is the accretion disk.
- Why is there
sometimes a ring around the moon? Is it useful for weather forecasting?
Contrary to what some people think, the ring around the moon is
not due to failure of the moon to use the proper
laundry detergent. Rather, the ring is caused by the reflection
of moonlight off of ice crystals in high, thin clouds. The ring has a
radius of 22 degrees, which is set by the properties of water.
Most often, the ring around the moon (and also around the sun) is
caused by high, thin cirrus clouds -- sometimes too thin to be seen by
any other means. Cirrus clouds tend to precede weather fronts,
meaning that bad weather is likely ahead. So, it is often said that a
ring around the moon or sun indicates rain.
To the best of my knowledge, professional meteorologists do not use
the appearance of a ring to forecast the weather.
I found two cool pictures of the ring phenomenon:
Thanks to Marla Geha, Mike Kuhlen, and Greg Novak for helping to
answer these questions!