Karen Hill

If you are stargazing at the equator, your celestial horizon is at right angles to the celestial equator. The North Star hovers on your northern celestial horizon. All the stars seem to rise and set at right angles to your celestial horizon, and there are no circumpolar stars. The reason for this is because the … Read more

In the same way that surveyors measure distances of uncharted land, by creating three points of a triangle, astronomers measure the distances of the stars. Mathematically, using trigonometry, if you know the length of the baseline of a triangle and the arc of two of its angles, you can figure out the distance to the … Read more

There are two kinds of time in astronomy, solar time and sidereal time. Ordinary clocks, and most people on earth use solar time, based on Earth’s rotation and orbit in relation to the Sun. The foundation of sidereal time is Earth’s rotation only, in relation to stars other than the Sun. Neither method of keeping … Read more

The universe is a dynamic, evolving organism. It is commonly defined as the totality of everything that exists. It had a beginning, probably a sudden, massive expansion of matter, and it may have an end. We don’t really know. Meanwhile, it grows by means of further expansion; galaxies move farther and farther away from each … Read more

Cosmology is the study of the evolution and structure of the universe. The word Cosmology comes from the Greek words κοσμολογία, or “kosmos”, which means “universe”, and λογία, or “logia”, which means “study”. Cosmology includes big questions. How did the universe begin? Will the universe end? How will the universe end? How do the components … Read more

The most popular theory about the universe’s creation is called the big bang theory. It is based on the ideas of many scientists, especially Edwin Hubble, a famous twentieth-century astronomer. The big bang theory claims that the universe was created by a massive surge of energy and matter 10 to 20 billion years ago. The … Read more

The zodiac is a group of twelve constellations. Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, and Aquarius. They comprise a band of the sky through which the Sun travels. The Sun’s apparent path through the zodiac is called the ecliptic. If you imagine being able to see the Sun and stars … Read more

Without looking at star maps as a reference, you can estimate degrees of distance in the celestial sphere by using a couple of tricks. In general, if you hold your hand in a fist vertically at arm’s length, it measures about 10° against the sky. Looking from Earth, the distance between the two stars that … Read more

Stars have names, but just like people, stars can have official names and nicknames. For instance, the brightest star in the constellation Leo has long been called Regulus. But not every star has been given a nickname, and nowadays it seems impractical to do so. Stars in constellations are now classified from the brightest to … Read more

You may notice that when you are stargazing in your yard at 10:00 every night, the stars appear slightly to the west of where they were the night before. Stars rise 4 minutes earlier every night, and set 4 minutes earlier. Therefore, over the months, the starry night changes. Over the period of a year, … Read more

Many of the stars you will see in the night sky follow a path across the sky from horizon to horizon. Circumpolar stars, however, appear to circle the northern and southern celestial poles without ever rising or setting. This is because the celestial sphere appears to rotate on the axis between its poles, in fact, … Read more

If you were looking at stars from Earth’s North Pole, the terrestrial globe would line up precisely with the celestial globe. Your celestial horizon would mirror the celestial equator. The North Star would be at your zenith, straight above your head. All the other stars in the northern hemisphere of the celestial sphere would appear … Read more

Your celestial meridian is an imaginary line that begins at the northern point of your celestial horizon and travels over your head, through your zenith, and down to the southernmost point of your celestial horizon. The line makes a hemispherical arc from north to south through your zenith. There is no special term for the … Read more

When you look up into the night sky, you see only some of the stars on the celestial globe. Some are on the other side of Earth from you, beneath your celestial horizon. To orient, or line up, your stargazing position in reference to the celestial globe, you have to imagine where your celestial horizon … Read more

Fortunately for us, the North Star, also known as Polaris, will be the polestar, marking the celestial north pole, during our lifetimes. But this will not always be the case. Earth does not rotate smoothly on its axis. Imagine a spinning top. As the top slows down, it wobbles a little. Instead of its axis … Read more

Constellations are simply groups of stars that people have named for the sake of convenience. People have been mapping constellations for thousands of years, usually naming them after mythological characters or common items that the group of stars resembles. For instance, the constellation Leo looks like a seated lion. If you were to connect the … Read more

While stargazing, it is sometimes difficult to estimate distances in the sky. Here are a few helpful shortcuts. Raise your index finger in the air; its width is approximately 1°. Make a fist and raise it to the sky: the width is about 10°. If you spread out your fingers against the background of the … Read more

Just as we use a terrestrial globe to show geography on Earth, we can use a celestial globe to locate stars in space. All the visible stars are on the surface of the globe. When you are looking at a celestial globe, you need to imagine yourself inside the globe looking out. Since stars “move” … Read more

A celestial globe is marked with imaginary vertical and horizontal lines called declination (dec) and right ascension (RA). These lines correspond to latitude and longitude lines on a terrestrial globe. Just as you can locate a town by finding the intersection of its latitude and longitude, Sonora, Texas, sits approximately at 30° north latitude and … Read more

Right ascension, or RA for short, is the astronomical term for the location of an object on the celestial sphere when using the equatorial coordinate system. On the terrestrial globe we use longitude lines to express east-west location on Earth. The prime meridian and the International Date Line are the imaginary vertical lines splitting Earth … Read more

Declination (dec) on the celestial globe is like latitude on the terrestrial globe. The celestial equator mirrors Earth’s equator, halving the sphere into northern and southern hemispheres at 0°. The degrees of lines of declination increase north and south to 180° at each celestial pole, just like latitudes increase to 180° at the terrestrial North … Read more

Say you are standing in your backyard. If you look straight up over your head, that is your zenith. It is the intersection of the imaginary north, south, east, and west lines rising from the horizon of your field of vision. In general terms, the zenith is the direction pointing directly “above” a particular location, … Read more

When you are stargazing, the farthest you can see along the ground in any direction is called your celestial horizon. Just as the Sun rises over the horizon, so do all stars appear to rise and set over your celestial horizon. On a map, your celestial horizon corresponds to Earth’s horizon around you, but when … Read more

Astronomers can measure temperature in space by studying various electromagnetic radiation waves. In the United States, the Fahrenheit (F) scale is usually used to measure temperature. Most of the rest of the world uses the Celsius (C) scale. Scientists, however, use the Kelvin (K) scale, also known as absolute temperature. In 1848, Lord Kelvin, also … Read more

While Isaac Newton’s laws of motion and gravity still have validity as a working model for basic astronomy, physicists rely on Albert Einstein’s complex relativity theories in their advanced work. The special theory of relativity, or STR for short, is the physical theory of measurement in inertial frames of reference proposed in 1905. In Einstein’s … Read more

Microwaves are not only found in ovens. In 1964, scientists Arno Penzias and Robert Wilson recorded microwaves emanating from every direction in the universe. Penzias and Wilson determined that their source was probably the entire universe. The waves came from everywhere. The only source that can be considered everywhere at once is the universe, if … Read more

Unless you are blinded by the bright lights of a city, the nighttime sky will show you bright lights of its own. The lights in the sky are objects emitting electromagnetic waves that we see as light. Lumped together, these objects, mostly stars, planets, satellites, comets, galaxies, and meteors, are often called celestial objects, or … Read more

There are many ways to look at the night sky. The most obvious is to stand outside and look up. On a clear, dark night, the sky, what astronomers call the celestial sphere, appears filled with individual points of light. Only by studied, careful observation can you distinguish a star from a planet or one … Read more

Even though we know that the heavens do not rotate around Earth, it only appears so because Earth is rotating on its axis, astronomers find this image useful in locating stars, planets, galaxies, and comets. The starry sky looks like a dome high above Earth. Imagine this dome completely surrounding Earth, as if Earth were … Read more

Traditionally, people have thought of the stars as immovable and permanent, but they do move. We know the stars move because we know that the universe is expanding. But Earth is also moving in the expanding universe, so the movement of the stars is imperceptible, you can’t see it. “Wait a minute,” you might say. … Read more

Light is a familiar everyday phenomena that we take for granted. When the Sun or other stars shine, we see light. When we turn on a lamp, we see light. Technically, light is an energy disturbance in the air. Oscillating electric and magnetic fields radiate energy in waves. The wavelengths and frequency fall in the … Read more

There are several types of electromagnetic radiation such as, radio waves, infrared light, visible light, ultraviolet light, and X-rays. Some radiation from elements can be deadly. The bombs that the United States dropped on Hiroshima and Nagasaki, Japan, at the end of World War H, were hydrogen and plutonium bombs, respectively. The radiation from other … Read more

The word “telescope” generally refers to an optical telescope. But instruments designed to collect radio waves are called radio telescopes or radio dishes. Radio telescopes are basically directional radio antenna used in radio astronomy. Radio telescopes operate in the radio frequency portion of the electromagnetic spectrum where they can detect and collect data on objects … Read more

Infrared rays are long, heat-intensive wavelengths that we cannot see. Telescopes outfitted with heat-sensitive reflectors, instead of mirrors or lenses, gather infrared rays, which are reproduced visually by special photographic equipment. The infrared image of an object is similar to what you see if you put your hand against a very cold window. An outline … Read more

Radiation is energy emitted by a source such as a planet, star, or your own body. We measure radiation in wavelengths. Radiation travels in rising and falling motions called waves, like waves in the ocean. Wavelengths are measured from the tip of one wave to the tip of the next. Our eyes can register light … Read more

We often refer to so called pure light as white, but light comes in a variety of colors depending on the length and speed, or frequency, of its waves. When light travels through a prism, or a solid, multisided chunk of glass, it is separated into its various colors, known as the spectrum. This separation … Read more

Many objects usually emit more than one type of radiation simultaneously. For instance, the Sun radiates everything from gamma rays to radio waves. People emit radio waves, but we can’t detect those waves without the proper receiving equipment. You’d have to use a radio telescope to do so, but all the other radio waves bouncing … Read more

Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves certainly have a lot to do with radios and astronomy. People cannot see radio waves without re-creating them in a visual format, such as by electronically digitizing the waves in a computer, which in turn … Read more

You have probably seen a rainbow, or light separating into colors through a cut-glass window. You can make your own color spectrum reflect on a wall or ceiling this way. Lean a mirror against the far end of a shallow pan filled with water, the whole mirror does not have to be underwater. Position the … Read more

All radiation travels outward from its source in a wave-like pattern. The term ray is commonly used to describe the rapid, short waves of the upper part of the electromagnetic spectrum: gamma rays, X rays, ultraviolet rays, light rays, and infrared rays. A ray is the straight line along which the waves travel. Slower radiation, … Read more

The astronomical unit (AU) measures the relatively short distances within our solar system. One AU is the mean distance from Earth to the Sun, about 93 million miles. We can say that Pluto’s average distance from the sun is 40 AUs rather than saying it is 3.72 billion miles. The units of light-years and parsecs … Read more

Perhaps the most common measure of space is the light-year. Strangely enough, a light-year can measure both time and space. Many people believe that time and space are inseparable, simply two sides of the same coin. It takes time for light to travel. Light’s unvarying speed in space is about 186,000 miles (297,600 km) per … Read more

Much of astronomy deals with the physical laws of motion and gravity, which Sir Isaac Newton described in the seventeenth century. Newton’s laws of motion are: 1. Every body of matter remains at rest until it is moved by an external force. 2. When an external force compels an object to move, that object will … Read more

A parsec (pc) is a measure of incredibly large astronomical distance. The word comes from the phrase parallax second. Parallax means the change in a star’s relative position in the sky when viewed from different places, and second refers to the smallest measurement of the change, stars move in arcs measured in seconds. Imagine a … Read more

The electromagnetic spectrum is the scientific name for all of the types of known radiation in the universe. From shortest (highest frequency) to longest (lowest frequency) wavelengths, they are: Gamma rays, X rays, ultraviolet rays, visible light rays, infrared rays, and radio waves, which include microwaves, television and FM radio waves, shortwaves, and AM radio … Read more

At the beginning of the seventeenth century, Hans Lippershey, a Dutch eyeglass maker, discovered that when he looked through two lined-up lenses at different ends of a long tube, distant objects appeared closer and larger. He was probably not the only one to make this discovery, but his name has come down through history. Lippershey … Read more

Galileo built many telescopes, each more powerful than the last, and in 1609 he was the first to use the instrument, called “Galileo’s ladder”, to study the sky. The great astronomer Sir Isaac Newton modified Galileo’s telescope by using mirrors instead of lenses. A musician and amateur astronomer named William Herschel, who discovered the planet … Read more

Let’s say we are looking at the Moon through a refracting telescope. Light from the Moon is gathered by the objective lens, which sits at the open end of the telescope. As the light enters the lens, it is bent just as light bends when it goes through water or a prism. The light bends … Read more

Most of what we know about the universe comes from the study of light from space objects. The general purpose of a telescope is to gather light from objects in the sky, making them larger, brighter, and clearer so that we can study them in detail. Two major types of optical telescopes are used to … Read more

The size of the objective lens in a refracting telescope, or the primary mirror in a reflecting telescope, and the distance the light travels through the telescope’s tube are the most important elements in determining the power of the telescope. The diameter of a lens or mirror is called the aperture; the larger the aperture, … Read more

Everyone relies on numbers to express distance, but when distances are immense, too many digits have to be used to make sense. For example, the distance between Los Angeles and New York is 14,710,080 feet, or 2,786 miles (5,280 feet = 1 mile). We don’t have to use as many digits in miles as in … Read more

Galileo Galilei seemed to be larger than life. Born in Pisa, Italy, in 1564, at the culmination of the Renaissance, Galileo was not just the first person to focus a telescope on the stars; he also turned the view of the world upside down. Galileo was a master of astronomy, mathematics, physics, philosophy, and publicity. … Read more

The acceptance of the fact that Earth rotated around the Sun was a long time coming. In 1543, the Polish astronomer Nicolaus Copernicus published De Revolutionibus, which stated that the planets revolved around the Sun. His theories, however, disagreed with the teachings of the Roman Catholic Church, and the church was the most powerful social … Read more

Besides mapping the locations of stars in the sky, astronomers also determined which stars were brighter than others. The Greek astronomer Hipparchus, a predecessor of Ptolemy, first classified stars according to their brightness. He listed six categories of brightness by magnitude. Magnitude defines how bright the stars appear to be from Earth. A star’s magnitude … Read more

Before the invention of the telescope in the early seventeenth century, astronomy was based on observations made by the naked eye. First, people mapped the positions of stars and planets in the sky. Most cultures had their own systems for mapping the sky, but astronomy as we know it today has its roots in classical … Read more

A reflecting telescope uses mirrors, whereas a refracting telescope uses lenses to gather light. Light gathered into a reflecting telescope first travels the length of the telescope’s tube until it reaches a mirror placed at the tube’s end. The light bounces off that mirror, called the primary mirror, and journeys back along the tube to … Read more

Astronomy is the study of matter and processes that exist primarily beyond Earth’s atmosphere. It covers the whole universe, the heavens, the celestial sphere, from microscopic atoms to the vast cosmos. Astronomers study the heavenly or celestial bodies such as planets, stars, comets, galaxies, nebulae, and intergalactic material to determine how they formed, how they … Read more