Questions excerpted from Observatory queries:

General astronomy

Palomar specific



I saw something bright/cool/weird up in the sky. What was it?

Probably a meteor, an airplane, a satellite, a rocket launch, or Venus, depending on what you saw:

Bright stationary lights are usually one of the brighter planets -- visit the Sky & Telescope website to find out where the planets show up on any particular month. Also, look at the object through a small telescope or binoculars. If it's oval or crescent-shaped, it's Venus. If it has a few small faint stars in line with it, this is Jupiter and its moons. If you can see rings sticking out to either side, then its (of course) Saturn.

A slowly moving light is usually just an airliner (especially if it's blinking or there's more than one light) or a low-orbit satellite. Slightly more exciting, if you're in the Southern California region, are the rocket launches which go up from Vandenburg from time to time, and leave behind strange contrails.

A fast streak of light is likely a meteor or fireball. Fireballs sometimes change colors and/or leave smoke trails behind. The Fireball Data Center and International Meteor Organization have more useful information, as well as a report form, in case you got a good fix on the sighting, and want to report it to professional meteor analysts.

Over the past several months, we've had an increasing number of reports of sun glints from Iridium satellites. The reflections off of the Main Mission Antennas of these low-orbit commsats appear as bright flashes in the sky, lasting some 10 to 20 seconds, and getting as bright as -8th magnitude. The Satellite Visibility Home Page will let you calculate the time and position of these events for any location on the globe, so you can see whether something you sighted was an Iridium flash, and predict when future flashes will happen.

If what you saw fits none of these explanations, let us know. Try to include the date and time, your location, the direction (north/south/east/west) and how high up in the sky it was, and its position relative to known constellations or the moon. It's unlikely that any of the Palomar telescopes happened to have seen it, since they only look at tiny little patches of the sky at once, but we still might be able to figure out what it was that you saw.


Could you please inform me of the recent televised publicity surrounding an asteroid. Is there really an asteroid on its way to our planet? If so what type of catastrophe are we looking at?

Well, we don't know of any major impacts in the near future, but that doesn't mean that some big piece of rock won't come hurtling out of the sky tomorrow. Chances are very very slim that it will be tomorrow, but over many thousands of years, you're pretty much guaranteed that something will come. It's sort of like auto accidents -- chances are slim that you'll get into a wreck on any given day, but over their lifetimes, most people get into at least one. So it's not something that you should worry about excessively, but you should play it safe -- wear your seatbelt and remain vigilant while driving.

In the case of asteroids, meteors, and all the other junk whizzing around the inner solar system, "vigilance" means accurately measuring and cataloging the many thousands of asteroids that pass through the Earth's orbit regularly, so that we will know in advance if we're in danger. The main asteroid belt is too far away to worry about, and deflections by a comet, as occur in recent TV and movie depictions, are really quite unlikely. (To continue the automotive analogy, you normally don't worry about a car plunging from an overpass and landing on your head -- it's the cars in the lanes around you that pose the most danger.) Many of these "near-Earth asteroids" have been observed by the SpaceWatch and NEAT programs, and the orbits predicted many years into the future, but there are many more that we don't know about. Hopefully, the recent publicity will prompt more public and government support of these monitoring programs.

And what do we do if we detect an inbound asteroid? This is still an open question. Any sort of laser or ICBM system would likely fragment the rock instead of destroying it entirely (just as in the TV show), so a better option would be to attach thrusters to it and nudge it just enough to miss Earth. Carl Sagan covers this topic in detail in his book Pale Blue Dot.

As for the magnitude of the catastrophe, there's a whole range, depending on the size of the object. Fortunately, the bigger rocks are rarer. To stretch the analogy just a little farther, fender-benders are pretty common, serious injury crashes less so, and 30-car pileups are rare (but they get all the headlines). Earth gets hit by many small chunks every day (some of which are seen as shooting stars), but these pose no threat. Rocks up to 10 meters in size come down every few decades, and may cause some local damage, but nothing serious. 100-meter objects hit every thousand years or so, and make craters like the one in Arizona. It takes a 1-kilometer object to have real global consequences, but these only happen on million-year timescales. You can find out more at this impact hazards site.

The bottom line: It's nothing to lose sleep over. But it is something our species has to plan for in the long run, or else we end up like the dinosaurs.


I've read about a direct alignment of seven planets on May 5, 2000 A.D., which will provide enormous gravitational pull upon the earth, and cause many catastrophic events to occur on the earth on that day. Is there any truth to this?

There indeed was an unusual alignment of the six innermost planets around May 5, 2000 with Earth on one side of the Sun, and Mercury, Venus, Mars, Jupiter, and Saturn roughly lined up (within about 25 degrees of each other in the sky) on the other side. However, the gravitational effects of this alignment were tiny. The Sun and the Moon really dominate the gravitational tug on the Earth -- the Moon because it's so close, and the Sun because it's so much more massive than the rest of the solar system put together. The rest of the planets are a negligible addition to this. So although this was a fairly rare alignment, we were not able to see it (the planets were all be up during the daytime), and it didn't cause tidal waves, reverse the earth's spin, or trigger earthquakes.

For a more detailed discussion, see Phil Plait's page at the Bad Astronomy website.


How often do astronomers use telescopes?

The Hale Telescope is used nearly every clear night of the year by a wide variety of astronomers. Individually, they only use telescopes for a few nights a year. In order to make the most of the limited telescope time, astronomers spend weeks preparing observing runs. This includes determining the best order and times to observe target objects and making backup plans for unpredictable weather. After the data have been collected, a great deal of time is spent on analysis. It often takes months or even years to turn the raw observations into scientific results. Astronomers spend most of their time on this analysis and researching others' work to fit results into a big picture. Some astronomers also develop and build instruments for the community to use. Below you see some astronomers at work.

Teaching, 117K
Documenting, 122K
Documenting Research
building, 145K
Building Instruments
analyzing, 141K
Analyzing Research


Which (Dipper) constellation is the largest? Besides the obvious, I was thinking that if the Little Dipper was further away, or the angle of view was different, that is if I were to put each Dipper into a container, which would need the larger container?

You are certainly correct in thinking that the apparent size of astronomical objects varies depending on their distance from us -- for instance, the Sun is much larger than the Moon, but they appear to be the exact same size from Earth, because the Moon is so much closer. It's hard, however, to apply the same logic to constellations, because the stars that make up a particular constellation are not necessarily near each other. Some may be relatively nearby us, and some may be very distant, and they simply appear to lie in roughly the same direction (from our point of view). So since a constellation is not really an "object", it's tough to say what its actual size is. The best we can do is talk about the angular size, as seen from earth.

(Carl Sagan's Cosmos, both the book and the TV show, talk about this effect a little. The TV show does a simulation of what the Big Dipper would look like if we moved to different perspectives -- it gets bent all out of shape.)


I understand that when we look at stars we are looking at "fossil light," because the light took many years to travel through space to us. Does that mean that someone very far away from us could look at us and watch history unfold? Could all that information be contained in light emanating from the earth?

The light is out there, but most of it's so distorted and scattered by now, after passing up through the atmosphere and then spreading out across interstellar space, that it would take astonishly advanced technology to reconstruct it into a coherent image. Plus, I bet a lot of history took place indoors. :-) The only easily-retrieved "fossil light" out there is probably our TV broadcasts, which have been spreading out now for about 50 years. If we have any nearby neighbors out there, the first they'll know of us will probably be the initial episodes of "I Love Lucy"...


How can something be 12 billion light years away in a 14 billion year old universe?

Upon first glance, it would seem that the light has been traveling for 12 billion years from something that started off 12 billion light years away in a 14 billion year old universe. That would leave only two billion years for the object to separate 12 billion light years from us, which means it would have to travel 6 times faster than the speed of light (on average), which is against the "law" of relativity. This seems to be a paradox. The answer is that space itself expands. This can be explained using a two-dimensional analogy to try to explain the effect (which actually occurs in three dimensional space). This analogy just illustrates the point - it is not to meant to be taken literally.

Picture a balloon that is deflated. Now, draw two dots 2 centimeters apart on the surface of the balloon. Now, pretend light travels at a speed of 1 centimeter per second on this balloon. Let's say at some time, a signal gets emitted from one of the "dots". (This is analogous to the source emitting light 12 billion years ago.) Now, since the universe is expanding, we need to blow up the balloon while the light is travelling.

Initially, when the light was released, the objects were only 2 centimeters apart, so it should only take 2 seconds to see the light at the other "dot". But, the universe is expanding, which we model by blowing up the balloon. Let's say you blow up the balloon at a rate that adds 0.8 centimeters to the distance between these dots for each second the universe ages. So, this means that one second after the light was emitted, the distance between the dots is now 2.8 centimeters. But light has already travelled one centimeter (since light travels at 1 cm/sec), so it has 1.8 cm left. Two seconds after the light was emitted, the distance between the objects is 3.6 (= 2 * 0.8[expansion] + 2[initial]) cm, but the light has travelled 2 cm. Continuing this, you'll find that it takes 10 seconds for the light to travel between the dots, even though they were only separated by 2 cm originally.

So, when the light is observed in the second dot (by us), it appears to have travelled 10 seconds and be 10 seconds old, even though the objects started only 2 cm apart. Let's say, for the sake of argument, that these objects (the dots) were 2.5 seconds old when the light went off. (All I've done is pretend that the universe expands at a constant rate, and after 2.5 seconds, two objects will be 2 centimeters apart.) 10 seconds later, the light is received in another galaxy. So, in the second galaxy, they will say "the dot is 10 light-seconds away and the universe is 12.5 seconds old." This is very similar to the apparent paradox, only the numbers are different. The idea is that space itself is expanding. It's not that objects are pushed apart by the energy from the Big Bang. Objects are being dragged along by the expanding space. So, they aren't being pushed, they're being pulled. Light is not immune to this pulling, and that's how we measure redshifts and get distances. The distance between the two galaxies increased between the time the light was emitted and received, which is why something can be 12 billion light years away in a 14 billion year old universe.


I want to learn more about astronomy. What should I do?

There are a lot of good astronomy books aimed at people of all ages. Books about constellations and the night sky are popular and there are many comprehensive guides to the sky including The Peterson Field Guide to Stars and Planets by Menzel and Pasachoff, and SkyGuide by Chartrand and Wimmer. Consult your local library, bookstore, observatory, or planetarium.

If you are interested in teaching yourself introductory astronomy, two good books are Astronomy: A Self-Teaching Guide by Dinah Moche, and Cosmos by Carl Sagan. Numerous well-written textbooks are available, usually written for first year college students (Astronomy Today by Chaisson and McMillan). Books about Palomar facilities and scientists include: The Perfect Machine by Ronald Florence, First Light by Richard Preston, and The Astronomers: Companion to the PBS Television Series by Donald Goldsmith. In addition, Sky and Telescope and Astronomy magazines do a good job of presenting astronomy to the public and can refer you to other sources of information. The weekly PBS series Star Gazer and two PBS miniseries, Cosmos and The Astronomers, do excellent jobs of explaining astronomy. There are occasionally astronomy segments on the PBS show NOVA. Many other science magazines (Discover, Science, Science News, Scientific American and science sections of newspapers (The New York Times on Tuesdays) frequently have astronomy articles. There are also a variety of good astronomy web pages on the World Wide Web. In addition to popular search engines (Google, Yahoo, HotBot, Excite, etc.), which often have astronomy-related links, we can recommend sites titled:

"Bad Astronomy" Devoted to clearing up many common astronomical misconceptions.

"Astronomy Picture of the Day" Daily astronomy pictures with explanations, relevant links, and a large image archive.

"Astronomy Cafe" Many astronomy articles and links. "The web site for the astronomically disadvantaged."

"Ask the Astronomer" An astronomer answers your questions! This site includes an archive of thousands of answered questions.

If you are considering becoming an astronomer, consult universities with astronomy or physics departments. A strong background in math and physics is very important, so get a head start!


I'll be visiting the observatory, and just want to take a quick peek through one of your high-powered telescopes.

Except for the occasional special occasion, none of the telescopes have anything that you can peek through. These days, very little professional astronomy is done through eyepieces -- computerized sensors like CCDs and IR arrays are much more sensitive and quantitatively precise. These digital imaging devices are attached to the telescopes in locations previously occupied by the observer's eyepieces. So, there's no longer any place to look through the telescope, as you can see in the two pictures below, which show the back ends to two Palomar telescopes. The one exception to this rule is on the 60-inch telescope, which is outfitted with an eyepiece once a month during the summer for special tours from the Fleet Museum in San Diego.

Splendid views of the night sky from the Palomar area are also available by attending an "Explore the Stars" star party with the Orange Country Astronomers. Although unaffiliated with the Observatory, they run monthly observing sessions at the nearby Observatory Campground, and their telescopes will probably give you a much better view of most celestial objects than any of the Palomar telescopes would.

200-inch, 141K
60-inch, 71K


What is the Hale (200-inch) telescope's magnification?

A telescope's magnification is determined by comparing the focal length (the distance from the mirror/lens to the focus) of the eyepiece to the focal length of the mirror or lens system. Since there is no eyepiece on the telescope, magnification has no meaning. However, the size of an image on a CCD is determined by the effective focal length of the mirror system. Since there are several different mirror configurations possible, the focal length depends on the observer's setup. The largest field of view (FOV) and, consequently, the least detailed images occur when observing at prime focus, at the top of the telescope. There, the FOV is half a degree (30 arcminutes) in diameter -- big enough to see the entire full moon at once. The smallest FOV, 1.4 arcminutes, occurs at Coude focus (no longer used).

The size of the smallest details visible in any image from a ground-based telescope is limited by atmospheric turbulence and is called the "seeing." Turbulent air effectively smears light out, preventing infinitely sharp images. If there were no atmosphere, the size of the telescope would determine the smallest resolvable details. This limit is caused by light bending around solid surfaces and is called the "diffraction limit."

Three images of the Moon, corresponding to the fields of view at prime focus (left, courtesy UCO/Lick), Cassegrain focus (center, courtesy Goshen Astronomy club), and Coude focus (right, courtesy Charles Genovese and the Lunascan Project). The rightmost two images show the Tycho crater (identifiable by many radial white rays in the bottom center of the full moon). These two images were taken during different moon phases, explaining the different crater shadows.

Full Moon, 600K Tycho crater and rays Tycho crater


Can the moon be observed with the Hale telescope? And if so, is it possible to see the landing sites of the Apollo moon missions and possibly the equipment they left behind? They told me at the Hubble web site that the moon is too close and too bright for observation by the Hubble telescope.

The 200-inch is rarely pointed at the moon, probably for the same reasons as the Hubble -- the full moon is some 60000 times brighter than even the brightest stars, and although you wouldn't set anything on fire (I estimate the total power collected to be about 0.02 watts), you'd certainly stress the highly-sensitive imaging systems on the scope unless you took special precautions.

Even if we could point at the moon (perhaps putting a huge Mylar filter over the end of the tube), the blurring effect of the atmosphere ("seeing") and of the telescope itself ("diffraction limit") would limit the detail we could resolve. The smallest angles we can distinguish through the atmosphere are about 1 arcsecond (1/3600 degree), which at the distance of the moon, covers a little under 2 km -- not enough to see any signs of our prior visits. Even if we could eliminate the blurring of the atmosphere (through advanced techniques known as "adaptive optics"), there is still an inherent limitation of 0.025 arcsec for the 200-inch scope, which comes to 50 meters on the lunar surface, not quite enough to see the LEM base or rover tracks.


Do any of the Palomar telescopes have the capability to see "detail" on the International Space Station? By "detail" I mean solar arrays, modules, maybe even an EVA crewman. If Palomar could see such detail, could any of the telescopes track fast enough to stay up with an object in low earth orbit, say at 150-200 miles altitude?

That second point is the crucial one, actually: objects in LEO are whipping along pretty fast, while the Palomar scopes are designed to track objects moving at the sidereal rate -- rising and setting as the earth rotates. The 200-inch, in particular, is a hefty beast, and getting it to track that quickly would take a lot more mechanical power than is available. The best you might do is to aim the scope ahead of the projected path of the satellite, and take a few snapshots as it streaks through the field of view.

The amount of detail visible would be limited (as it is for astronomical objects too) by the blurring effect of turbulence in the atmosphere, known as "seeing". With normal imaging, we can't resolve details any smaller than 1 arcsecond (1/3600th of a degree), which at 150 miles range, comes out to just about 1 meter, so most of the features of the station would be recognizable. In fact, you could see just as well with a much smaller telescope, since the station would be plenty bright, making the huge light-gathering area of the 200-inch unnecessary. Folks have imaged Mir & the Shuttle with 12-inch scopes, using special sat-tracking software -- see pg 86 of the August '96 Sky and Telescope.


How far can the Hale telescope see?

This depends on how long you look and how bright the object is. There are three things that determine how far away you can see something (such as a galaxy): its energy output, the exposure time of the image, and the amount of light that is blocked along the way. The first two factors are linked together by the "inverse square law." This law tells you that the further something is from you, the fainter it appears. If you move something three times further away, it appears nine times dimmer. So, if you can only see things that are brighter than a specific level, you can see a galaxy (which contains one hundred billion stars) much further away than you can see a single star, because the galaxy emits more light than a star. This means that in order to see intrinsically faint sources, they must be relatively close, while bright objects can be seen much further away.

Astronomers usually get around this nearness limitation by taking longer exposures. The amount of light you see on an image is proportional to how long you look. The longer you look at a region, the more light you collect, allowing you to see fainter objects. This effect is demonstrated in the left two pictures. The bottom image shows a 10 second exposure of a sparse star field, while the top image shows a 120 second exposure of the same region. Clearly you can see fainter stars using a longer exposure. (Both images are negatives.) If something (like dust) blocks some of the light from your source, it will be harder to see objects of interest. Since there is a lot of dust between us and the galactic center, we cannot see many stars when we look in that direction. The right picture illustrates what dust does to starlight. In this picture, some of the faint stars in the dark region are actually more luminous and closer to us than the brighter stars outside this region. However, most of the light of the fainter-looking stars is obscured by intervening dust and never reaches the camera, making the stars appear fainter (and redder) than they really are.

So, why is a bigger telescope better? Because it gathers more light than a smaller telescope. The Hale telescope, with a collecting area of about 31,000 square inches, can see the same thing that a smaller telescope (with an area of 3,100 square inches) can see, only it takes less (one-tenth of the) time to see it. So, building a bigger telescope means you save yourself a lot of time!


Long exposure, 133K 2MASS dust, 424K
Short exposure, 119K


I'd like to know the origin and meaning of the word Palomar. Is it someone's name? Is it the name of the mountain peak the observatory sits on? Or is it a mythological name?

It was originally the name of the mountain where the observatory is located. "Palomar" translates to "dove haven" in Spanish and/or the local Indian dialect. There was apparently once a large population of doves in the vicinity.


Who gets to use the telescopes?

Half of the time is available to Caltech, one quarter to NASA/JPL (Jet Propulsion Lab), and one quarter to Cornell University. The observatory is owned and operated by Caltech, while the other institutions help with the finances and instruments. Specific users (usually astronomers or planetary scientists) are determined by each institution's Telescope Allocation Committee (TAC), which meets twice a year. Astronomers apply to their local TAC, which decides 1) if the project is scientifically useful, 2) if the project is feasible, 3) if the project is a good use of the resources, and 4) if the time is available. Astronomers are assigned nights according to these priorities. Usually the 200-inch has 50% more hours requested than available (compared to 1000% on the Hubble Space Telescope) so there is some competition between astronomers. If weather is bad on your assigned night, tough luck! You have to reapply for more telescope time.


Why is it so dark inside the dome?

The dome is not lit because light bulbs emit twenty times more heat than light. In order to light the dome, you would need many bulbs, which would significantly heat up the air. The dome air must be kept at nighttime air temperatures all day long. This minimizes the blurring caused by different temperatures in the air around the telescope. You can see this blurring effect over the pavement in hot parking lots, in the air above hot barbecues, and behind engine exhausts (below, right).

On the left is a picture of the instrument bay at Cassegrain focus with the cage lights on. On the right, you see airplanes' exhaust ports heating the air behind them, creating temperature differences in the air. This causes the blurry trail seen on the right side of the building.


Cassegrain cage Plane exhaust distortion view, 159K


Where can I get a Palomar picture/poster/slide of a particular galaxy/ planet/nebula/cluster? What about pictures of the telescope itself?

The Palomar Gift Shop has a few pretty pictures, but it's not equipped to deal with mail-order requests. Be sure to check out our on-line astronomical images and desktop wallpaper. There are also many non-Palomar sources of astronomical imagery -- the Astronomical Society of the Pacific has a very nice online catalog with posters and slides galore.


Where's the Caltech Virtual Planetarium that I saw on the TV show Millennium?

We have no such web facility -- the show's writers were taking a little creative liberty. Try the star charts at What's Up This Month, or Que tal in the Current Skies. There are also many planetarium programs for different computers listed at STScI's Astroweb listing.


I'm a college astronomy major, and I'm interested in doing summer research at the Observatory. What's available?

The Observatory doesn't run any programs on its own, but Caltech has an excellent summer program called SURF, which brings students from other schools to campus for 10 weeks to do research with Caltech faculty. To apply for this program, you'll have to have a specific project mapped out with a particular member of the faculty, so browse the faculty roster and read through the "bluebook" articles and personal webpages to see what sort of research they do, and then contact them directly.


Can you drive to Palomar in an RV?

We've seen some pretty big RVs up in the visitor lot (and also in the state campground just a few miles down the road from the observatory), so it must be possible to get there in one. The tricky bit is probably on the really twisty part of the S6 just south of the observatory (see this map), but you can probably circle around to Lake Henshaw and find a straighter road from there.


I am trying to find a friend of mine who works at the observatory.

Check the department listings of staff and researchers.

Page created by David Vakil 





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