Frequently Asked Astronomy Questions
As with any hobby, amateur astronomy can seem a little intimidating for those who are just beginning. Beginners can have a lot of questions about the the astronomy hobby and about astronomy in general. This astronomy faq page contains answers to some of the most frequently asked questions encountered in the amateur astronomy hobby, ranging from general astronomy to the Sun, the Moon, stars, and the Solar System.
We will be adding more questions and answers as time goes by. You can click on one of the questions below to jump directly to the answer, or browse at your leisure by scrolling down the list. If you have an astronomy question that is not answered here, please feel free to contact us by e-mail and we will try our best to answer the question.
General Astronomy Questions
The Solar System
Reader Asked Questions
General Astronomy Questions
What is the difference between astronomy and astrology?
Believe it or not, this is the most asked question that astronomers encounter. Many people do not understand the difference. In ancient times, they were considered one and the same. But the two disciplines were separated during the Age of Reason in the 17th century. Astrology is a practice of using the locations of the planets to look into a person's personality or predict the future. It is not a science and is considered a form of divination. By contrast, astronomy is the scientific study of the universe. Astronomers observe the objects in the night sky to try to determine their composition and learn more about the origin and structure of the universe.
Do I need an expensive telescope to enjoy astronomy?
Many people hesitate to get involved with astronomy because they believe it requires expensive equipment. The only thing you really need to enjoy the night sky is your eyes, a dark viewing location, and some patience. To get a better look at things, a pair of binoculars can provide a really good view. Many people will be surprised how many more stars and objects they can see with a decent pair of 10X binoculars. They collect much more light than the human eye and will bring much dimmer objects into view. You can even see Jupiter’s moons with binoculars. A simple camera tripod to steady the binoculars is also a good idea, since your arms can get tired very quickly.
How does a telescope work?
The primary purposes of a telescope are to gather light and magnify an image. The aperture (opening) of a telescope is larger than that of the human eye and therefore, can gather much more light. This enables us to see dim objects that are too faint to see with the naked eye. The larger the aperture of the telescope, the more light it can gather. Telescopes also use a series of lenses and/or mirrors to magnify the image, enabling us to see more detail.
Why can't I see very many stars at night?
If you live near a big city, you may not be able to see a lot of stars. The reason for this is light pollution. Dust and water vapor in the atmosphere reflects the bright city lights back down towards the ground. This “light pollution” tends to be brighter than some of the dim stars and other deep sky objects, essentially hiding them from view. To truly appreciate the night sky, you must get as far away from city lights as possible. There is no more beautiful sight then the band of the Milky Way stretching across a dark sky. We can all help to combat light pollution by convincing our local authorities to use more efficient light fixtures that shine the light on the ground and block it from going up into the sky.
Where does space begin?
Space is defined as the area above the Earth’s atmosphere. But there is no specific boundary since the atmosphere gradually thins out as you move farther away from the Earth. However, NASA awards astronaut status to anyone who flies above 50 miles (80 km).
Why is the sky blue?
This is another question that gets asked a lot. The blue color of the sky during the day is caused by scattered sunlight. The white light from the Sun is composed of all the colors of the rainbow. During the day, the molecules in the air scatter the blue light from the Sun more that the red light making the sky appear blue. In the evening, however, we see the red and orange colors because the blue light has been scattered away from our line of sight.
Why is the sky dark at night?
Believe it or not, there is no easy answer to this question. Scientists have observed that if the universe was infinitely large, and contained an infinite number of stars, then the night sky should actually be as bright and as hot as the surface of the Sun. But this obviously is not the case. This little brain teaser has come to be known as Olber’s paradox, named after the German astronomer who tried to solve the problem in 1823. The most likely explanation is that the universe is simply not old enough and the observable part of the universe contains too few stars to fill the sky with light. Thus, the night sky is dark.
What is the speed of light?
Light travels at a constant speed of 186,262 miles per second (299,792,458 meters per second). Since the speed of light is constant, it can also be used to measure vast distances. Distances between objects in space are measured in light years. One light year is equal to the distance light travels in a year, which is just under 6 trillion miles (10 trillion kilometers). The speed of light is considered to be the ultimate speed limit in the universe. Scientists believe that it is impossible to travel faster than light because any object traveling at the speed of light would have to achieve infinite mass.
Did galileo invent the telescope?
Many people believe that Galileo invented the telescope, but they are wrong. It was actually a man named Hans Lippershe from Holland who assembled the first telescope. Several years later, Galileo became the first person to use a telescope for astronomical observation. With his early telescope, Galileo observed the craters on the moon, the rings of Saturn, and the moons of Jupiter.
How old is the Earth?
All though the exact age of the Earth is difficult to determine, scientists believe it to be about 4.54 billion years old. This age has been estimated by measuring the amount of radioactive decay in rocks and minerals on Earth, and by observing the elements that exist in the Sun. The age of the Earth is believed to be the same as the age of the Sun and the other planets in the Solar System since they all formed at the same time from the same raw materials.
How big is the Earth?
The mean diameter of the Earth is 7,918 miles (12,742 kilometers). Believe it or not, the Earth is not a perfect sphere. The rotation of the planet causes it to bulge slightly at the equator. The diameter at the equator is 7,926 miles (12,756 kilometers), while the diameter at the poles is only 7,900 miles (12,720 kilometers). Likewise, the circumference of the Earth at the equator is 24,902 miles (40,075 kilometers) while the circumference at the poles is only 24,860 miles (40,008 kilometers). Because the Earth is flatter at the poles and wider at the equator it is actually considered to be an oblate spheroid instead of a perfect sphere. However, if the planet were to stop spinning on its axis, gravity would pull it into a nearly perfect sphere.
What is the Earth made of?
The Earth is actually layered much like the layers of an onion. The outer layer is called the crust. The crust is composed mainly of iron, oxygen, silicon, magnesium, sulfur, and nickel with trace amounts of other elements. The next layer down is the mantle, composed mainly of magnesium and iron. The inner-most layer is called the core, which is composed of two parts. The outer core is believed to be a liquid iron and nickel alloy while the inner core is a solid mass of iron. The inner core is thought to spin within the liquid outer core, creating a magnetic field that protects the Earth from solar radiation.
How much does the Earth weigh?
The Earth is actually weightless because it exists in space. But it does have mass. If we were able to weigh the Earth in its own gravitational field, it would weigh approximately 13.2 septillion pounds. That is 13,200,000,000,000,000,000,000,000 pounds (6,000,000,000,000,000,000,000,000 kilograms). This number assumes we are weighing the planet in Earth's gravitational field. It would weigh more or less on other planets, depending on their mass. But wait, there more. The Earth is actually gaining mass. It is believed that our planet is gaining about 40,000 metric tons of mass each year from space debris that impacts the surface and burns up in the atmosphere.
Why How fast does the Earth rotate?
The Earth rotates once on its axis each day. Because it is so large, it is moving incredibly fast. Its speed, however, depends on the location you are measuring. At the equator, the speed is 1,038 miles per hour (1,670 kilometers per hour). The speed decreases as you travel further north or south. The middle latitudes of the United States and Europe are moving at between 700 and 900 miles per hour (1,125 to 1,450 kph). At the the poles, it is hardly moving at all. At the south pole, you would take an entire day just to rotate once in place.
How fast does the Earth move around the Sun?
The Earth takes one year (365 days) to orbit the Sun. Since it is 93 million miles away from the Sun, it has to be moving extremely fast to cover this vast distance. As it orbits the Sun, the Earth moves at an average of about 67,000 miles per hour (107,300 kilometers per hour). This is only an average because the Earth's orbit around the Sun is not perfectly circular. It is actually slightly elliptical. When the Earth is closer to the Sun, it moves a little bit faster. When it is further away, it moves a little slower.
What is the terminator?
As the Sun shines on the surface of the Earth, it illuminates one half of the Earth's surface. The other half is in darkness, or night. The dividing line between day and night is known as the terminator. The terminator line passes through any given point on Earth two times a day, once during sunrise and again during sunset. Because of the tilt of the Earth's axis, the polar regions are an exception. They spend parts of the year in perpetual day and night. The terminator is not a sharp line on Earth. It is actually kind of blurry because the atmosphere scatters the sunlight. This is why we have dawn and dusk before sunrise and sunset.
How fast does the terminator move?
The terminator line moves across the surface of the Earth as the Earth rotates on its axis. Therefore, the speed that the terminator moves is the same as the speed that the Earth rotates. At the equator, the speed would be 1,038 miles per hour (1,670 kilometers per hour). The speed decreases as you travel further north or south.
How thick is the Earth's atmosphere?
The Earth's atmosphere is an extremely thin sheet of gas that surrounds the planet. All though the planet is nearly 8,000 miles (12,875 kilometers) in diameter, the atmosphere is only 62 miles (100 kilometers) thick. This thin sheet of gas is all that separates us from the cold vacuum of outer space. All though some gas molecules extend out as far as 300 miles (480 kilometers), the majority of the atmosphere's mass lies below 62 miles, which is generally considered to be the edge of space.
What is the Earth's atmosphere made of?
The Earth's atmosphere is made of a variety of different gasses. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.4% carbon dioxide, and trace amounts of other gasses such as neon, helium, and methane. It also contains about 1% water vapor, which forms clouds and storms.
What is the Moon made of?
Science is still learning about the composition of the Moon. We know a lot about its surface, but the inside is still a mystery. The outer layer of the Moon is known as the crust, and is composed mainly of silicon, oxygen, magnesium, iron, calcium, and aluminum. Below the crust is the mantle, which is believed to be made primarily of the minerals olivine, orthopyroxene and clinopyroxene. The Moon's mantle is thought to contain more iron than the mantle of Earth. At the center of the Moon is the core, which is believed to be composed of metallic iron with small amounts of nickel and sulfur.
How far is the Moon from Earth?
The distance between the Moon and the Earth averages 238,857 miles (384,403 kilometers). Since the Moon’s orbit is not a perfect circle, its distance varies. At its farthest point, known as apogee, it is 252,080 miles (405,686 km) away. At its closest point, known as perigee, it is 225,621 miles (363,104 km) away.
Why does the Moon shine?
The Moon shines because the light from the Sun shines and reflects from the Moon’s surface. What we think of as Moon shine is actually just reflected sunlight.
Why is the Moon larger when it is close to the horizon?
Although the Moon looks much larger when it is low in the sky near the horizon, this is actually just an optical illusion. It is actually the same size as when it is directly over head. This illusion has been known since ancient times and also happens with the Sun and the constellations. This same illusion works on mountains and tall buildings as well. They appear larger at long distances than they do at closer distances. The reasons for this are complex, but they have something to do with how our brains interpret the sizes of large objects on the horizon. If you don't believe this is only an illusion, you can compare the size of the Moon near the horizon to the size directly over head by holding your finger out at arm's length and comparing the sizes of the Moon with your finger.
What causes the phases of the Moon?
The Moon goes through phases because it is traveling around the Earth. One half of the Moon is always illuminated by the Sun. As the Moon circles the Earth, different amounts of the illuminated part of the Moon are facing us. These phases range from Full Moon (when the Moon is on the opposite side of the Earth then the Sun) to a New Moon (when the Moon is between the Sun and the Earth. It takes about 29 and a half days for a complete cycle, which equals one complete orbit of the Moon around the Earth.
Can you see the flag on the Moon with a telescope?
This is a question that astronomers get asked a lot. Unfortunately, the equipment left behind by the Apollo missions is tiny in comparison to the size of the Moon. Ground-based telescopes, especially those owned by amateur observers, are not capable of resolving objects this small at such extreme distances. Extremely large telescopes could theoretically catch a bright spot of sunlight reflecting from some of the moon landing equipment, although they would not be able to observe the equipment directly.
What is a blue moon?
The answer to this question is a bit complicated. The most recent and most popular definition says that a blue moon is the second of two full moons occurring in the same month. Since the lunar cycle is 29 days and most months have 30-31 days, we eventually find a situation where a full moon occurs at the beginning and the ending of the same month. There is also a second, older definition of a blue moon. This one defines a blue moon as the third full moon in a season with four full moons. Normally there is one full moon each month, so a season such as summer would usually have three full moons. The reason for this is complex, and has to do with the ancient Christian ecclesiastical calendar. This calendar was used to determine important dates such as Easter. Each of the usual 12 full moons of the year had a name associated with the time of year in which they usually occurred. In a year with 13 full moons, the extra full moon was referred to as a blue moon so the calendar could stay on track.
How far away is the Sun?
The average distance from the Sun to the Earth is 93 million miles (149 million kilometers). Because the Earth’s orbit around the Sun is not a perfect circle, it varies. At its closest point to the Sun, known as perihelion, the distance is 91 million miles (146 million km). At is farthest point, known as aphelion, the distance is 94.5 million miles (152 million km).
How big is the Sun?
The Sun is an average-sized star that is 865,000 miles (1,392,000 kilometers) in diameter. It is so large that you could fit the planet Earth inside it well over a million times. The Sun actually makes up about 99% of the entire mass of the Solar System. The remaining objects, including all of the planets, moons, comets, and asteroids compose the other 1% of the Solar System.
How hot is the Sun?
The core of the Sun is extremely hot at about 27 million degrees Fahrenheit (15 million degrees Celsius). The surface of the Sun is much cooler than the core, at about 9,900 degrees F (5,500 degrees C). For some strange reason, not yet completely understood by scientists, the Sun’s outer atmosphere is hotter than its surface. Known as the corona, its temperature reaches 5 million degrees F (2.7 million degrees C).
How long does it take the light from the Sun to reach Earth?
The light from the Sun travels at the speed of light, 186,282 miles per second. Since the Sun is about 93 million miles from Earth, it takes the light about 8.4 minutes to reach us. This means that when you look up at the Sun, you are actually seeing it the way it looked 8.4 minutes ago. To give you an idea just how close we are to the Sun, the light from the next nearest star, Proxima Centauri, takes 4.3 years to reach the Earth.
What are sunspots and why do they appear dark?
Sunspots are temporary areas on the surface of the Sun that are cooler than the surrounding areas. They are caused by intense magnetic activity that inhibits convection and reduces the surface temperature. Sunspots appear dark on images of the Sun taken with filters because the filter significantly reduces the brightness of the Sun overall. If you could observe a sunspot by itself, away from the rest of the Sun, it would actually be blindingly bright.
What is the solar wind?
The solar wind is a stream of charged particles that are ejected from the Sun in all directions at a very high rate of speed. This constant stream of particles can interact with planets by slowly blowing away their atmospheres. The Earth has a strong magnetic field that deflects the solar wind around the planet and protects our atmosphere. Sometimes the particles from the solar wind can interact with gases in the upper atmosphere of the Earth, causing them to glow. This phenomenon is known as the aurora borealis (northern lights) in the northern hemisphere and the aurora australis in the southern hemisphere.
The Solar System
How many planets are there in the Solar System?
This used to be an easy question. Many of us grew up learning about the nine planets, starting with Mercury and ending with Pluto. But recent discoveries of other Pluto-type objects in the outer Solar System began to call Pluto’s planet status into question. Finally, in 2006, the International Astronomical Union (IAU) decided to change the official definition of a planet. Pluto was reclassifies as a dwarf planet, leaving the total number of planets in our Solar System at eight.
What is the largest planet in the Solar System?
Of the eight planets in our Solar System, the largest is Jupiter. This giant planet is over a thousand times larger than the Earth, and is composed mostly of hydrogen gas. The famous giant red spot on Jupiter is a giant storm system that has been raging for several hundred years and is actually twice the size of the Earth.
What is the smallest planet in the Solar System?
The smallest planet in the Solar System used to be Pluto, with a diameter of 1,441 miles (2,320 kilometers). But in 2006, Pluto was demoted and reclassified as a dwarf planet in. Now Mercury is the smallest planet in the Solar System with a diameter of 3,032 miles. ( 4,879 kilometers).
How old is the Solar System?
Astronomers believe that the Solar System is about 4.6 billion years old. They have determined this age in part by studying meteorites. It is believed that meteors formed at the same time as the rest of the Solar System from a could of dust and gas. When meteors fall to earth as meteorites, a technique called radioactive dating can be used to calculate how old they are. Astronomers also believe the Sun is about middle aged, which means it should continue to shine for about another 5 billion years.
What exactly is a meteor shower?
A meteor shower is an event where a large number of meteors appear to radiate from a common point in the night sky. The meteors are cause by streams of debris left over by comets. This debris is usually no larger than a grain of sand. As the debris enters the Earth’s atmosphere at a high rate of speed, the friction causes the gases to glow. This glowing trail of ionized gas is known as an ionization trail. Meteor showers occur at the same time every year as the Earth passes through the debris field. Meteor showers can be stronger if the comet has recently passed by leaving a fresh stream of debris.
What are asteroids and where do they come from?
An asteroid is a small, rocky or metallic body found in orbit around the Sun. Most of the asteroids in our Solar System are located between the orbits of Mars and Jupiter in an area known as the asteroid belt. They are thought to be the left over remains of a planet that was either destroyed or never fully formed. Asteroids can range in size from a few feet to several miles in diameter. Occasionally, gravity can cause an asteroid to change its orbit and send it on a path towards the inner Solar System. If the asteroid crosses the orbit of Earth, it is known as a near-Earth asteroid, or NEA. NEAs have a small chance of eventually colliding with the Earth. Meteor crater in Arizona is an impact crater nearly a mile across that was caused by an asteroid about the size of a city bus. It is believed that an asteroid about 5 miles in diameter may have been responsible for the extinction of the dinosaurs.
What are comets and where do they come from?
Comets are relatively small Solar System bodies composed of ice and dust. Due to this unusual composition, many astronomers refer to them as “dirty snowballs”. It is believed that most comets originate in an area at the outer edge of the Solar System known as the Oort cloud. Occasionally, gravity will disturb a comet’s orbit and send it on a new course toward the inner Solar System. Comets with highly elliptical orbits like this are known as periodic comets, and they return to the inner Solar System at a regular period. The most famous periodic comet is comet Halley, with a period of 76 tears.
Why do comets have tails?
Comets are probably best known for their long, luminous tails. These tails are actually plumes of dust and gas that are ejected from the comet as it nears the Sun. Comets are composed of frozen ice and dust. If a comet’s orbit takes it close to the Sun, the solar radiation will cause the volatile materials in the comet to vaporize, carrying some of the dust along with them. As the Sun shines on this vaporized material, known as the coma, it begins to glow. The solar wind pushes the material out away from the comet. Because of this, a comet’s tail always points away from the Sun. Comets will usually have two tails, one formed from ionized gas, and the other formed from dust reflecting the sunlight.
What is a star?
A star is a gigantic, luminous ball of heated gas, or plasma, held together by gravity. They are formed mainly of hydrogen and helium. Stars burn helium in a process called nuclear fusion, where helium atoms are fused together under enormous pressure and temperature to form helium. This process gives off an incredible amount of energy. Stars are very large. The Sun, which is the closest star to the Earth, is so large that you could fit a million Earths inside it.
How far away is the closest star?
This is actually a trick question. The closest star to the Earth would be the Sun, at a distance of about 93 million miles (149 million kilometers). The closest star outside our solar system would be Proxima Centauri. It is located about 4 light years from Earth. That is over 23 trillion miles (38 trillion kilometers).
Why do stars seem to twinkle?
The twinkling of stars is caused by instability of our atmosphere. As the starlight passes through the atmosphere, the movement of the air bends the light slightly and makes the stars twinkle. If you could view the stars from outside the atmosphere, like the space station or the Moon, they would not twinkle.
How many stars are visible in the night sky?
The number of stars visible in the night sky depends on many factors, such as the clarity of the atmosphere, the time of the year, and the amount of light pollution. But on a good night, far away from city lights, you should be able to see about 2,000 stars with the naked eye. Astronomers have calculated that about 6,000 stars should be seen from the darkest locations.
How many stars are there in the universe?
The universe is unbelievably huge. There are billions of galaxies and each galaxy contains billions of stars. The latest estimates from astronomers say that there are a staggering 300 sextillion stars in the known universe. That is a 3 followed by 23 zeros, or 3 trillion times 100 billion. That represents several stars for every grain of sand on Earth.
What is the brightest star in the sky?
The brightest star visible in the night sky is Sirius, located in the constellation Canis Major. With an apparent magnitude of -1.46, it is nearly twice as bright as Canopus, the second brightest star. Apparent magnitude is a measure of a star’s brightness as seen from Earth. The lower the number, the brighter the star. Some of the planets appear like stars and can be brighter than Sirius when they are close to the Earth. Jupiter can have an apparent magnitude of -2.6 when it is close.
How long do stars live?
The life cycles of stars differ greatly and depend mostly on the star’s size. Large stars burn their fuel faster while smaller stars burn it more slowly. The largest stars burn their fuel so fast that they only last for a few million years. Average sized stars like the Sun live for about 10 billion years. Smaller stars, such as red dwarfs, burn their fuel so slowly that they can live for trillions of years. Our Sun is believed to be about 4.5 billion years old. It should shine for about another 5 billion years.
How do stars die?
Just as the lives of stars depends on their size, so does their death. Stars exist in a state of equilibrium because the gravity pulling in on them equals the pressure pushing out. When a star begins to run out of fuel, the outward pressure decreases, and gravity wins. The effect of gravity on the star depends on its size. Smaller stars will eventually lose their outer layers and shrink their cores to form white dwarfs. Larger, more massive stars will experience much more violent deaths. Gravity crushes these stars so fast that a shock wave is created, resulting in a massive explosion known as a supernova. What remains behind depends on the mass of the star. Large stars will form extremely dense objects known as neutron stars. The largest and most massive stars will experience such a tremendous crush of gravity that they will literally be crushed out of existence into what are known as black holes.
What exactly are constellations?
Constellations are arbitrary groupings of stars that seen to form pictures. Ages ago, ancient people looked up at the night sky and thought they could see patterns. Since different constellations are visible at different times of the year, people used these patterns to tell what time of the year it was. Today, we use the constellations to map the night sky and help classify the locations of objects.
How many constellations are there in the night sky?
Throughout the ages, different cultures saw different patterns and images in the stars. Thus, the constellations have changed over time. Today, we divide the sky into 88 different constellations. Astronomers use them to classify objects in the sky. Every star or other object in the night sky falls into one of these 88 constellations.
Are all of the stars in a constellation the same distance from us?
No. Most of the stars in a constellation have no connection with one another. They are simply chance alignments of the stars. The stars are all different distances from us, but because they are so far they all appear flat as seen from Earth. If you could travel far out into space, the constellations would look completely different from your point of view.
Are the constellations permanent?
All of the stars in the sky are moving in relation to the Earth. Since they are so far away, it takes thousands of years to notice their movement. But eventually the movement of the stars will make today’s constellations completely unrecognizable.
How old is the universe?
This is a question that has puzzled astronomers for many years. The most recent estimates put the age of the universe at between 13 and 20 billion years. However, as new discoveries are made with the Hubble space telescope and other new technologies, these numbers may continue to be revised.
How big is the universe?
The latest estimates say it is about 156 billion light years across. Since one light year equals about 6 trillion miles, that is a very big number. But since we can only see so far, we may never know exactly how big the universe is. Some astronomers even believe the universe is infinite in size. And there may be even other universes out there beyond ours.
How did the universe begin?
The most widely accepted scientific theory today suggests that the universe began about 14 billion years ago from an infinitely small, dense, and hot state that expanded rapidly. As it continued to expand and cool, hydrogen gas formed into stars and eventually formed into galaxies. This theory is known as the big bang theory and helps to explain why the universe appears to be expanding today. Astronomers have observed that galaxies are moving away from each other. The farther away a galaxy is, the faster it is moving away. The big bang theory states that it is actually the fabric of space that is expanding. Matter is simply going along for the ride like dust on the surface of a balloon. As the balloon expands, the dust gets farther and farther apart.
How will the universe end?
Scientists used to think that the combined gravity from all of the matter in the universe would eventually slow its expansion and cause it to contract back into an infinitely small, dense state as it existed before the big bang. But now, most agree that there is not enough matter in the universe to slow its expansion. This means that it will continue to expand until all of the stars eventually burn out. Eventually, matter will decompose and all that will be left is a cold, dark void. That may sound depressing, but it will take at least 100 trillion years.
How many galaxies are there in the universe?
This is a question that continually puzzles astronomers and is subject to frequent revision. The reason for this is because we can only see so far with the instruments we have available to us today. The most recent estimates by astronomers suggest that there are about 200 billion galaxies in the known universe. However, as new telescopes and new technologies emerge, this number is almost sure to be increased as we gain a better understanding into the true size of the universe.