The University of Texas at Arlington

The University of Texas at Arlington

UTA Planetarium

UTA Planetarium

Ask the Astronomer Q&A

Tag: "moon"

  • Can the Moon be seen everywhere on Earth?
  • Yes. At the same moment, the Moon looks pretty much the same shape, wherever you are on Earth. The difference made by travelling a few thousand miles to the other side of the world is very slight because the Moon is dozens of times further away than that. So, Full Moon happens at the same moment everywhere on Earth.

    (Tags:  earth  moon)
  • Explain why high tides occur along the line joining the Earth and Moon, even on the opposite side of Earth from Moon and why low tides occur at 90 degrees to the line.
  • The moon's gravitational force pulls on water in the oceans so that there are "bulges" in the ocean on both sides of the planet, which we call high tides. The Moon pulls water toward it, and this causes the bulge toward the Moon. The bulge on the side of the Earth opposite the Moon is caused by the moon "pulling the Earth away" from the water on that side. Low tides occur at 90 degrees from the Moon because the water in the ocean has been pulled away from these locations towards the bulges that create the high tides. Since the Earth is rotating, each location will experience two high tides and two low tides every day.

    (Tags:  earth  moon  tides)
  • Have astronauts ever been to ___?
  • Astronauts have never been anywhere besides the Earth and Moon (but there have been no Moon trips since 1972). There are plans to return to the Moon (possibly by 2020) and then astronauts may travel to Mars. A trip to the Moon takes about three days, but a one-way trip to Mars will take close to nine months.

    (Tags:  astronaut  earth  mars  moon  nasa)
  • I received an email saying that we will see Mars as big as the moon on August 27, 2010. Is this true?
  • No, this is not true. The emails started circulating in 2003 when Mars made its closest approach to Earth. It was closer than it had been in 60,000 years! But, because the distances between Mars and Earth is so great (at and average of 46.5 million miles away) and their size so small (Mars is only half the size of Earth), Mars can never look as large as the Moon. That year, it appeared as a very bright red dot in the sky. Mars is visible this month in the Western part of the sky after sunset. It will appear as a red dot low in the sky. If you want to read more about this email hoax and others we see, check out these great articles on our website!

    (Tags:  astronomy  earth  light  mars  moon  planet  solar system)
  • I was wondering if you could recommend a good telescope to get a better look at space. I don't mean a very expensive one, but a reasonable one to get a good look at the stars, planets.
  • If you are just beginning to use a telescope for enhancing your stargazing experience, we would recommend the Celestron FirstScope, which we have available in our gift shop for $60. This is a table-top telescope with a 3-inch diameter, perfect for viewing the planets and bright deep space objects, such as the Orion Nebula and the Andromeda Galaxy. This telescope is easy enough for children to use, but will last a long time. If you would like to get an intermediate or advanced telescope, I would recommend contacting the Texas Astronomical Society at They have a great online resource for finding the perfect telescope for your needs, and experts able to answer all your questions.

    (Tags:  astronomy  jupiter  mars  moon  planet  saturn  solar system  star  telescope)
  • If Astronauts are traveling in zero gravity, why can't we travel at the speed of light? Doesn't that mean they are weightless?
  • Actually, no. The effects of gravity are felt everywhere, even in space. When an astronaut experiences weightlessness or zero-G they aren’t actually feeling a lack of a gravitational field, but instead they are experiencing free-fall. The shuttle, in Earth’s orbit, is actually free-falling around Earth, so we don’t feel a pull directly towards the surface. That is why an astronaut - and all objects – float in space. Also, gravity is what causes us to have weight, not mass. Our weight can fluctuate depending on where we are in the universe. On Earth an adult may weigh 150 pounds, but on the moon, which has a smaller gravitational field, the same person would way only 50 pounds. But his mass, the amount of material that makes the adult, does not change wherever he is. You can determine your mass by dividing out 9.8m/s^2 (which is the pull we feel from the Earth – its gravitational effect) from your weight as measured on Earth.

    (Tags:  astronaut  earth  gravity  moon  physics)
  • If I were to fast-forward into the future, and I was setting up a base on Callisto, what might be the best place to do that? Would somewhere like in the Lofn crater be a good place to put it? If so, what would the area look like? Would I be able to see Jupiter from there? Would the ground feel and look like snow? Would the thin atmosphere carry any sort of sound?
  • Callisto is a tidally locked moon which means that the same side is always facing Jupiter (like our Moon). Unfortunately, the Lofn crater is located on the far side the moon (56˚ S and 23˚W) so your base would never see Jupiter. The surface of Callisto is covered by ice (water ice), carbon-dioxide and silicates; so it wouldn’t feel like snow, it would feel like hard ice. The thin atmosphere is so tiny; you can almost consider there is no atmosphere. It is surprising that this tiny moon has an atmosphere; however, it is not stable. The atmosphere is actually running away and is probably being replenished by the surface carbon-dioxide. Such an atmosphere wouldn’t carry any sound.

    (Tags:  jupiter  moon  physics  solar system)
  • If the weight of our planet does not fluctuate... Does this include matter and mass ON the planet as well?
  • It depends on how the weight is calculated. If we estimate the density as a function of the radius of Earth, we can calculate the total mass up to its surface, not including the matter on the planet (air, trees, animals, etc.). If we calculate the mass from Newton’s 3rd law - using the Sun’s or the Moon’s mass (assuming that their mass are well known) – then we will figure the mass of Earth with everything on the planet. However, there will not be a significant difference as the air mass and the matter on the surface is negligible. Moreover, Earth’s weight is not constant. Every year, 100,000 pounds of dust and debris falls on Earth from the sky. Of course, this increase is negligible and will not affect the planet.

    (Tags:  earth  moon  physics  sun)
  • Is everything we see in the universe on fire or able to be seen because it is reflecting light from something on fire?
  • The answer kind of depends on what you are referring to. Take the planets of our solar system for example. They do not create their own light, they are just reflecting the Sun’s light back to us, the same is true for our Moon. On the other hand, the Sun, stars, galaxies and nebulae are all creating their own light. But I wouldn't say that they are on fire. It is true that astronomers talk about stars “burning” but we aren't referring to a fire like you may create in your fireplace or grill. What’s actually happening is Hydrogen atoms in the center of the star (like our Sun) are fusing to form Helium atoms. This creates an incredible amount of heat and light. The Sun is more than 10,000 degrees Fahrenheit!

    (Tags:  hydrogen  light  milkyway galaxy  moon  planet  star  sun)
  • Is it possible for a satellite to maintain an orbit on the far side of the Moon (always hidden from Earth), given the complicating effects of Earth's gravity? If it is possible, what would the semi-major axis be?
  • No, it would not be possible for a satellite to orbit completely hidden from the Earth. To keep a satellite on the far side of the moon, it would have to orbit the Earth, not the Moon, every 28 days. Using Kepler’s law of planetary orbits, we can find the semi-major axis of the object with this equation (p^2/(a^3). Where a is the semi-major axis and p is the period. We know we want the period to be the same as the moon’s, 28 days. Which means that the semi-major axis would be equal to the moon’s semi-major axis. So the only way for the satellite to remain on the far side of the moon, is to place it on the moon itself. It is possible for the object to share the moon’s orbit (by trailing behind or ahead of the moon), but then we would be able to see it.

    (Tags:  earth  gravity  moon  satellite  telescope)
  • Since it takes the moon about 28 days to orbit the earth, does that mean a solar eclipse happens somewhere every 28 days?
  • If the moon orbited in the same plane as the ecliptic – Earth’s orbital plane – we would have two eclipses every month. There’d be an eclipse of the moon at every full moon. And, two weeks later, there’d be an eclipse of the sun at new moon for a total of at least 24 eclipses every year. But the moon’s orbit is not the same as the ecliptic. It is inclined to Earth’s orbit by about 5 degrees. Twice a month the moon intersects the ecliptic at points called nodes. If the full moon or new moon is appreciably close to one of these nodes, then an eclipse is not only possible – but inevitable. Even though the moon’s orbit is inclined to that of Earth – and even though there’s not an eclipse with every full and new moon – there are more eclipses than you might think. There are from four to seven eclipses every year. Some are lunar, some are solar, some are total, and some are partial.

    (Tags:  earth  eclipse  moon  sun)
  • The center point of the Moon's rotation is Earth, the center point of the Earth;s rotation is the Sun, the center point of the Sun's rotation is the black hole at center of our galaxy; does that mean that the universe has a central point that can be pinpointed provided we had the instruments to measure this vast distance?
  • The center of the universe would be the location where the Big Bang happened. Since that time, all matter has moved uniformly away from this point in all directions. This expansion of the universe is still happening today and is actually accelerating. So instead of orbiting some central part of the universe, the universe is actually moving away from the center. Scientists are trying to discover what is causing the acceleration of the expansion of our universe.

    (Tags:  big bang  black hole  earth  milkyway galaxy  moon  sun  universe)
  • The Lunar Reconnaissance Orbiter (LRO) scientists mentioned the Moon's craters that have ice in them are probably charged up by static electricity from interacting with the solar wind. Is there a way to measure the voltage from some probe or other?
  • This might be a good one to try to send to the LRO scientists because this will be depend on the amount of ice, as well as electromagnetic field strength caused by the solar wind. Unfortunately, static electricity can’t be measured by probes unless the voltage carrying medium is grounded and discharged. For example, we can’t measure the static electric voltage building up in clouds. However, if we knew the amount of current that was transmitted to a lightning rod by the cloud, then we would be able to estimate the amount of static electricity carried in the cloud just before the lightning.

    (Tags:  moon  nasa  satellite)
  • What is the size and brightness of the Sun as viewed from the different planets in the Solar System?
  • We have a few simple calculations that you can use to find the size (in degrees) and the brightness of the Sun on each planet. In fact, these calculations will work for the brightness and size of the Sun from anywhere in the universe, as long as you know the distance of the object from the Sun in AU. To find the size of the Sun in degrees: a = 0.5/distance (in AU) where a is the size in degrees and 0.5 is the size of the Sun as seen from Earth. To find the brightness of the Sun as compared to its brightness as seen on Earth: M = 2.5 * log(distance)^2 – 26.74 Where M is the apparent magnitude of the Sun and 26.74 is the magnitude of the Sun as seen on Earth.

    (Tags:  earth  jupiter  mars  moon  planet  pluto  saturn  solar system  star  sun  venus)
  • Why doesn't Mercury have moons?
  • Most moons are captured asteroids or rocks left over from the formation of the solar system. Since Mercury is so close to the Sun, asteroids and comets that come close by are captured by the Sun’s gravity. They are either pulled in to the Sun and burned up or they go around the Sun as comets. Mercury doesn’t have as strong a pull of gravity as the Sun, so it is unlikely that it would be able to redirect the path of any incoming objects to turn them into moons.

    (Tags:  asteroid  comet  gravity  mercury  moon  sun)
  • Will NASA ever get to Mars knowing that it's so far away?
  • I think what you mean to ask is not whether NASA will get to Mars, but will NASA send astronauts to Mars, and I think the answer is yes. We have been sending robotic probes to Mars for decades. Several are on the surface testing the soil and sending beautiful panoramic images back to us on Earth. Others are in orbit, taking high resolution images of the surface from above. The problem with sending astronauts to Mars is that it would be a 1-3 year trip. It all depends on the technology we use and how close Mars and Earth are to each other in our orbits. NASA’s Constellation Program, which was started under the Bush Administration, was a project to take the best of the Apollo and space shuttle missions, and create a spacecraft that could return astronauts to the Moon, and maybe eventually to Mars or some near-Earth asteroids. But the project was over promised, and over budget, so it was removed from the NASA budget. But, the current NASA budget has money set aside for taking us to Mars or near-Earth asteroids, it is just hoping for some help from the commercial market. It may take 10-20 years for the new system to be developed, but we will get to Mars within our lifetime.

    (Tags:  asteroid  astronaut  earth  mars  moon  nasa)