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Ask An Astronomer

Do you have a question that you'd like to ask an astronomer? Send your question to astdept [at], and we'll respond to it and post it here. We can't answer every question we receive, but we'll respond to as many questions as we can.

Previous questions:

Why is it so cold on the surface of Mars?

Answered by Jeff Cummings.

There are two reasons why Mars can get very cold:

Mars is approximately one and a half times as far from the Sun as the Earth is, which means that it only receives approximately 44 percent of the energy from the Sun that the Earth does. This leads to cooler temperatures.

Compared to Earth, Mars has a very thin atmosphere surrounding it. An atmosphere can act like a blanket that traps in heat and doesn't let it escape away from the planet. So, since Mars's atmosphere is so thin it doesn't trap in the heat very well and this causes the temperatures on Mars to get extremely cold. [top]

Is Pluto a planet?

Answered by Jayce Dowell.

Discovered in 1930 by Clyde Tomball at Lowell Observatory, Pluto is currently classified as a planet by the International Astronomical Union. However, recent finds of objects at the outer edge of our solar system and discoveries of other solar systems have thrown this classification into question. First, Pluto is neither a large nor massive object. In fact, it is even smaller than our Moon. However other objects, such as the unimaginatively named 2003 UB313, have been discovered that are not only larger than Pluto but also further from the Sun. Some argue that if Pluto is called a planet so should 2003 UB313. Another oddity about Pluto that has some astronomers calling for it to be removed from the list of planets is that its orbit is quit different from those of the other eight planets. These planets all lie in roughly the same plane and have orbits that are almost perfectly circular. Pluto, on the other hand, has an orbit that is must more elongated and is inclined greatly relative to this plane. In short, Pluto’s orbit is quite unlike that of any other planet. Third, Pluto’s moon, Charon, is almost half the size of Pluto leading some astronomer to classify Pluto and Charon as more of a “double planet” system. This line of reasoning leads to Charon and other large objects, such as 2003 UB313, being classified as planets. Finally, we see from the over 150 planets discovered outside our solar system that our solar system seems to be part of the exception rather than the rule. This would lead to Pluto-like planets to being ejected through interactions with larger planets in these systems. In these cases, Pluto would never be classified as a planet. In fact, it may never even be seen.

If we buy into these arguments that Pluto is not a planet, then what is it? It would be a large asteroid or, depending on whether or not it has a large about of ice, a comet. In particular it would be part of class of objects known as Trans-Neptunian Objects or TNOs. TNOs are a collection of rocky and icy bodies that lie outside of the orbit of Neptune. This group of objects can be further divided into Kuiper Belt objects, such as 2003 UB313 and objects that lie even further away, the bodies of the Oort Cloud.

I believe that Pluto is a planet because of both its size and it distance from the Sun. My definition of a planet is a body with a diameter greater than 2000km that has an average orbital distance of less than 40 times the Earth-Sun distance that does not orbit a another body. This definition leads to Pluto being a planet, Charon being a moon of Pluto, and 2003 UB313 being a large Kuiper Belt object. This definition can be applied to other solar systems as well. [top]

What is the red spot on Jupiter?

Answered by Kevin Croxall.

The Great Red Spot is a huge storm which has been raging for years on Jupiter! It was first spotted in the seventeenth century and has been there ever since! This huge storm or vortex is larger across than the diameter of the entire Earth! It is constantly turning and spinning with winds of over 200 mph. [top]

How do people measure the distance planets are from the Sun?

One means that astronomers use to measure distances to nearby planets is Radio Detection and Ranging, or RADAR. In this method radio waves are transmitted from Earth and bounced off of the planet. Radio waves are a type of light we cannot see with our eyes. Since radio waves travel at the speed of light, the time it takes for radar signals to travel to the planet and back tells us the distance to the planet. That is, the time required for one half of the trip is multiplied by the speed of light to give the distance to the planet. This method only works for planets with solid surfaces.

The distances to the more distant Jovian planets may be determined through geometry. For a more advanced discussion of this visit the following website: [top]

What would happen if you landed on Jupiter?

Answered by Christian Johnson.

Assuming Jupiter has a solid core somewhere deep in its interior, the spacecraft would not survive the descent through Jupiter's atmosphere as it would quickly be crushed and melted by the high pressure and temperature at depths of only a few hundred miles. Theoretically, it would be possible to "float" in one of the cloud layers of Jupiter's atmosphere, but it would be like landing in the middle of a very powerful hurricane with wind speeds exceeding several hundred miles per hour. If you could somehow avoid being tossed around by the wind, you would still need a spacesuit to leave the spacecraft because Jupiter contains little oxygen and is composed of mostly hydrogen and helium, like the sun. It would also be more difficult to move around because Jupiter is more massive than the Earth, so you would weigh about two and a half times more than you do on Earth. [top]

Where is the Voyager I right now and what information is it giving you?

Answered by Scott Michael.

Voyager I is about 9.7 billion miles away from Earth right now. It sends us limited information about its surroundings and uses an ultraviolet spectrometer to measure the ultraviolet light output of stars in our galaxy. [top]

How is Uranus different from Jupiter?

Answered by Ryan Maderak.

Before we talk about Uranus and Jupiter, it will help to know some definitions. MASS is a measure of the amount of matter in an object. DENSITY is the amount of mass per unit volume in an object. For example, if you have a brick, and a block of metal the exact same size, the metal will be heavier. because it has more mass. But, because the metal has more mass in the same amount of volume (it takes up the same amount of space), that means it is more dense.

First, let's talk about how the two planets are similar. Uranus and Jupiter are both made mostly of gas. Each one has a large number of moons, and each one has rings (although they are very hard to see). Also, Uranus and Jupiter are both only a little bit more dense than water.

But, Jupiter is over 300 times more massive than the Earth (it would take over 300 Earth's worth of matter to make Jupiter), while Uranus is only about 15 times more massive than the Earth. Also, Jupiter is 11 times larger than the Earth in diameter, but Uranus is only 4 times larger than the Earth in diameter.

Jupiter is very warm, and is made mostly of hydrogen gas, while Uranus is colder, and is made of water, methane, and ammonia. Uranus has a core of rock and metal that is 10 times more massive than the Earth, but Jupiter has no core. Also, Uranus may have a layer of diamond on top of its core!

Jupiter spins very rapidly, which causes it to have very large storm systems and hurricanes like the Great Red Spot. Uranus, on the other hand, spins slowly, and has only very weak storms.

In short, Jupiter and Uranus may seem the same, but in fact are very different. [top]

Why did they name planets after gods?

Answered by Ted Maxwell.

So eight of the nine planets are named after gods from greek and roman mythology. Specifically, their names come from the Latin forms. Mercury, Venus, Mars, Jupiter, and Saturn were all observed by the ancient Greeks and their names come from that time period. Uranus, Neptune and Pluto were discovered much later, but in order to keep up the tradition, the discoverers continued to name them after the Roman gods.

Now the reason the greeks named them after gods is best understood by the fact that for the majority of scientific history, most people observing the heavens were not doing so strictly for scientific reasons. Even the most educated of these guys were mostly philosophers and not scientists the way we think of them today. Of course most people who looked at the sky were folks like you and me just trying to make sense of what they saw. For people who first noticed these planets (note that the term planet comes from the greek word for wanderers,) it would be only natural to attribute them to some miraculous source. In fact, in the greek tradition, both the Sun and the Moon were specifically considered to be gods. Many of the everyday people probably believed that the sun and the moon were gods, or at least caused by them. The same could also be true of the planets as well. On the other hand, many early philosophers considered the planets to be representative of the gods but not the actual gods themselves.

Now this may seem a different way of thinking to us, but the Greeks were not alone. Most early civilizations also named the planets after gods! It's all rather interesting, if you ask me. [top]

Why are there craters on Earth?

Answered by Kevin Croxall.

The simple answer is that space is a busy place!

Here is an image of all that is out there (that we know of)!

Solar System Inner

This includes the comets and asteroids which are near the Earth (the large circle with a + on the 3rd blue circle out from the center) . And the solar system was a lot more crowded when it first formed! Since there is so much out there, things run into each other every now and then. When something hits us, it leaves a crater. Like if you dropped a rock into some flour. One might wonder then why we don't have more craters like the moon does? There are a couple of reasons. 1) We have an atmosphere. If you have ever seen a shooting star, it is a small rock from space entering our atmosphere and burning up due to friction. They burn up so much that you would need a rock bigger than you car to actually survive all the way to the ground. 2) We have a changing surface. The surface of the Earth is constantly changing through volcanoes, Earthquakes, and tectonic plate motions, so occasionally craters are destroyed (there is no activity like this on the moon). 3) Erosion. Water and wind, which don't exist on the moon, wear down craters until we can't tell they were once there. 4) Most of the Earths surface is covered with water. Impacts which occur on water can still be devastating, but will not leave a crater unless the water is fairly shallow. [top]

Why do we study the planets?

Answered by Aaron Boley.

Planets and possibly their moons are the best places for harboring life as we know it. Even though only a small fraction of planets may turn out to be capable of supporting life, it is necessary to understand the formation of all types of planets because the formation of Earth-like planets may be linked in some respects to the formation of gas giants like Jupiter. If we know how planetary systems form and if we can characterize the requirements for life, we can set up models that use observational data to predict the frequency of habitable planets. [top]

What are the specifications of the telescope in the Kirkwood observatory? What magnification, aperture size, etc.?

Asked by Jim J., Bloomington, IN
Answered by Brian Brondel


Kirkwood Observatory's telescope is a refractor — meaning it has a lens at the top to focus the light rather than a mirror at the bottom. This style of telescope was once much more popular among very large telescopes, but newer telescopes are strictly reflectors.

Contrary to common misconception, the purpose of a telescope is not to magnify distant astronomical objects, but to brighten them. The telescope makes faint sources appear brighter by collecting all the light incident on its main lens (the objective) and focusing that light into a beam small enough to fit into your eye or onto an electronic detector. Telescopes with a larger lens can collect more light, so they can amplify the brightness of an image more effectively. The size of a telescope's lens is called its aperture. The Kirkwood telescope's aperture is twelve inches. This aperture is similar to the aperture of high-end commercial telescopes for amateur stargazers. The Kirkwood telescope was considered a cutting-edge instrument when it was built, but advances in mirror and lens making has made much larger telescopes possible. The largest optical telescope in the world today is the Keck telescope at Mauna Kea, Hawaii, with an aperture of almost 10 meters.

The magnification of a telescope is determined by the size of its eyepiece. The Kirkwood telescope has a magnification of 100x at its lowest-power configuration. The eyepiece can be changed out for a magnification of up to 1000x.

It's worth noting that many telescopes sold in department stores advertise magnifying power in the hundreds or thousands. These claims should be treated with skepticism. While it's easy to increase the magnifying power of a telescope, the higher magnification is not useful unless the telescope has a high optical quality and a sturdy mount. In general, telescopes of good quality can handle 50x magnification for each inch of aperture. A four-inch telescope, for example, can handle magnification up to 200x if it's well-built. When buying a telescope, it's wise to avoid products that make unreasonable claims about magnifying power, as these telescopes often lack in other important respects as well.

In general, the Kirkwood telescope is a high-quality instrument with a style of manufacture typical of the late nineteenth century. The telescope is built chiefly from iron and brass, although aluminum parts have also been added over the years. The glass was ground by John Brashears, who was one of the most talented lens makers in the world. The mount was built by Warner & Swasey Company, a widely recognized manufacturer specializing in astronomy equipment. [top]

How can I measure the bearing of sunrise with a compass?

The Naval Observatory sun azimuth table always calculates as North 360° minus sunset azimuth equals sunrise azimuth within a third of a degree, throughout the year. For example our local 21 June Solstice is listed as sunset 303.6°, sunrise as 56.5, so North 360°. minus 303.6 equals 56.4. Here in Saratoga Springs NY, the sun's [actual] azimuth at sunset 11 Apr. 06 was approx. 304°. The sun's [actual] azimuth at sunrise was approx. 104°. Using the Naval Observatory formula, North 360°. minus 304 should equal 56°. The actual calculation of 104° is a whopping 48° off! And the [actual] sunset azimuth today exceeds the government's own calculated azimuth more than nine weeks away! I simply used my Silva orienteering compass to get the approximate actual bearing for sunrise and set and compared them to the table calculated by the U.S. Naval Observatory website. What's going on?

Asked by S.V., Saratoga, NY

Answered by Aaron Boley

The azimuths given by the Naval Observatory are correct, and you can calculate the rising and setting azimuths of the Sun for a given day yourself! Let's first establish some coordinates and definitions. Azimuth is the angular separation of an object's projection on the horizon as measured eastward of true north. True north is the direction toward the northern pole of Earth's rotation axis, and it is different from magnetic north. The north magnetic pole is located at about 79 degrees north latitude, 104 degrees west longitude. We call the angular separation between true and magnetic north for some observer deviation.

The next coordinate we need to understand is declination. Declination is the angular separation of an object's projection onto the celestial sphere from the celestial equator, i.e., the projection of Earth's equator onto the celestial sphere. The celestial sphere is an imaginary sphere surrounding the Earth that provides a backdrop onto which objects in the sky are projected. When we look at an object in the sky we see its two-dimensional projection. As the Earth moves in its orbit about the Sun, the Sun's projection onto the celestial sphere moves. Because the Earth's rotational axis is tilted 23.45 degrees with respect to a perpendicular to Earth's orbital plane, the Sun changes declination throughout the year. During the equinoxes, the Sun is at 0 degrees declination, which means its projection is on the celestial equator. During the northern hemisphere's summer, the Sun's declination is 23.45 degrees, while it is -23.45 degrees for the northern hemisphere's winter. It is geometry and Earth's orbital motion that causes the rising and setting azimuth of the Sun to change locations as the year progresses. By using spherical trigonometry, one can calculate the rising and setting azimuth of the Sun, i.e., when the center of the solar disk is crossing the horizon, by only knowing your latitude and the Sun's declination with the formula

azimuth = arccos[ sin(declination)/cos(latitude) ].

I believe the error in your measurement is two-fold: First, as explained above, one must use true north when measuring the azimuth of an object. For New York, the magnetic deviation from true north is between about 12 and 16 degrees. This accounts for a large piece of the error. Second, I'm guessing that the compass you used does not have a clinometer, and that probably accounts for the rest of the error.

If you would like to measure the setting and rising azimuths of the Sun for a given day, I suggest that you try one of the following two methods: For the first method, use a compass as you originally did, but use a compass with a clinometer. Then, make sure you correct for the magnetic deviation in your calculation of the azimuths. The second method is a bit more complex, but it avoids using a compass. Set some foam board onto a table that is located where you have an unobstructed view of the rising and setting Sun. Tie some string around a pin and stick the pin into the center of the foam board. Use a meter stick to get a line-of-sight from the pin toward the spot on the horizon where the Sun rises. Once you have the line-of-sight, mark the line-of-sight on the foam board. Repeat for the setting Sun. Measure the angle where the lines intersect with a protractor. Make sure you measure the angle that sweeps out the north side of the foam board. Half of that is the rising azimuth of the Sun and 360 degrees minus the half angle is the setting azimuth, both within 3 degrees of that given by the Naval Observatory, depending on the care you take with your measurements. [top]

Will Mars gravity pull have an effect on ocean tides and other earth objects for the month of Aug. 2006?

Answered by Caty Pilachowski

An internet hoax has been circulating ever since the close Mars opposition in 2003.  It turns up every summer.  Mars will NOT be close to Earth on Aug 27.  It was in 2003.  Because of their different orbital periods, Mars and Earth are on the same side of the Sun about every two years.  The last opposition was late October in 2005, and the next one will be about 2 years after that.  This August, Mars  is on the far side of the Sun  from Earth.  This image shows the relative positions of the Earth, Mars, and the Sun on August 27, 2006. 

The effect of gravity goes as one over the square of the distance, and directly as mass.  At closest approach in 2003, Mars was still about 35 million miles away (if I remember correctly) compared to the Moon's distance of a quarter of a million miles.  Thus, just on distance, the tidal effect would be  1/140 squared, or 0.00005 (5 x 10^-5) of the Moon's.  Even with 10 times the mass of the Moon, Mars' gravitational effect on the Earth is still only 0.0005 (5 x 10^-4) of the Moon's gravitational pull at closest approach, and less when Mars is further away.

Image here

Regarding the description of the current positioning of the Poles of the Sun found at , which states, "this is due to the way in which the polarity changed during solar maximum. Instead of reversing completely, flipping north to south, the Sun's magnetic poles have only rotated at halfway and are now more or less lying sideways along the Sun's equator."

I understand from the quote that those were the measurements at the time of how the poles were magnetically positioning themselves as configuration. Has this configuration remained as an equatorial placement of the N/S poles still lying E/W? Are there further documents indicating the precise current positioning as actual measurements taken that I can in fact reference in the interests of scientific accuracy?

Answered by Caty Pilachowski and Kevin Croxall

We were able to answer this question with the help of solar astronomers John Leibacher and Jack Harvey at the National Solar Observatory.  The magnetic field of the sun is quite complex and variable. But the more distant one gets from the surface, the simpler it appears. Far away it resembles the field from a bar magnet. This simple approximation is used for comparison with solar wind measurements and other features in the distant heliosphere, where the ESA spacecraft Ulysses makes its measurements. Because the fields at the surface of the Sun are constantly changing, the strength and tilt angle of the hypothetical bar magnet used to approximate the real field  also constantly change.

At the times of weak solar activity, such as now, the poles of the hypothetical bar are oriented N-S, coincident with the Sun's rotation axis, and the strength of this hypothetical bar magnet is also at a maximum. As solar activity increases, the patterns of real magnetic fields at the surface of the Sun become stronger and more complex so that the hypothetical bar magnet field weakens and no longer describes the solar magnetic field at large distances from the Sun.  Eventually, the solar magnetic field reorganizes itself back into a bar magnet-like state at large distances from the Sun, but its polarity has flipped, north to south and south to north.  Of course, this behavior should not be thought of as a giant magnet inside the Sun actually changing strength and flipping 180 degrees, but rather is a mathematical approximation to the changing patterns on the surface of the Sun.

To answer the question directly, the period of time when the bar magnet approximation was lying in the equator, as reported in the press release that prompted this question, was brief, and the overall magnetic field re-established itself quickly into the expected, strong NS orientation. Thus, the Sun is behaving just as the solar astronomers think it should be. [top]

Are there extra solar sources of laser light that reach Earth?

The only astronomical lasers I know of are OH IR maser stars and masers that arise in similar, low density astrophysical environments.   Masers (Microwave amplification from stimulated emission of radiation) are similar to lasers but the light has a wavelength too large to be seen with our eyes.  The wavelengths of microwaves are typically a few centimeters or a few tens of centimeters.  Visible light has wavelengths that are more typically about 0.0005 millimeters.  The OH molecule is the most common source of maser light from astronomical sources, but other molecules are also known to produce masers.OH IR maser stars are stars near the end of their evolution that have bloated up to huge radii and that are losing mass via stellar winds blowing matter from the upper layers of the star back out into space.  Molecules of OH form from atomic oxygen and hydrogen in the material blowing away from the star.  Due to the geometry of the outflow, the OH molecules can form natural masers that amplify emission from these molecules at particular wavelengths of light.  These masers are detected with radio telescopes, and the radiation is very weak.  It has no effect on the Earth.

More information is available on the Wikipedia web page on OH masers.

See on the web. [top]