Units of Measurement

Welcome from the Opening Tour or from the Introduction to the Night Sky page. It is extremely important for every astronomy student to develop a familiarity with units of measurements which are used to compare celestial objects with us and each other.

Units of Short Distance

The simplest unit we use is the meter (m). American students are used to using English measurements for distance such as feet, yards, and miles. But the entire science community has shifted to using the metric system because it makes more sense and is based on a mathematical pattern more easy to understand. The meter is 39.77 inches, so it is just a bit longer than a year. A meter is broken down into smaller units by a factor of 10 with each reduction. It never ceases to amaze me at the extreme stubborn attitude of the American public to continue to use English measurements of inches, feet, yards, and miles which are seemingly unrelated in any simple mathematical pattern, and yet we continue. I hope you will find the metric system to be easier to relate to and follow, even if it is presently unfamiliar to you.

Below is a short table to reference submeter units.

Meter (m)

The standard unit of the metric system

Decimeters (dm)

1/10th of a meter or 0.1 m = 10^-1m

Centimeters (cm)

1/100th of a meter, or 0.01 m = 10^-2m

Millimeters (mm)

1/1000th of a meter, or 0.001 m = 10^-3m

Micrometers (mum)

1/1,000,000th of a meter, or 0.000001 m = 10^-6m

Nanometers (nm)

1/1,000,000,000th of a meter, or 0.000000001m = 10^-9m

Angstroms (A)

1/10,000,000,000th of a meter, or 0.0000000001m = 10^-10m

These small units of measurement are useful to astronomers when studying the properties of starlight at different wavelengths.

Units of Long Distance

The next unit is the kilometer (km). A kilometer is 1000 meters, and is 0.62 miles. In a track meet, runners who compete in the 1500 m run are actually running slightly less than one mile, and therefore the times are a bit faster. Kilometers are useful when measuring distances between Minneapolis and Honolulu, or between the Earth and the Sun. For instance, the Moon is an average of 384,000 km from Earth. But when we start looking at the distance to the Sun, we find that 149,600,000 km is a pretty huge number, and it takes up too much space on the page.

Therefore, astronomers have resorted to a simple unit of measurement called the Astronomical Unit. This is the average distance between the Earth and the Sun. The 149,600,000 km becomes 1 Astronomical Unit (AU). Now we have a nice reference unit to use when describing distances to other planets. Mars is 1.888 AU from the Sun, Jupiter is 4.3AU from the Sun, and so forth.

Astronomy students have little grasp of the distance between stars, and units such as the AU are completely impractical when measuring the distance to even our closest neighbor, Proxima Centauri. Astronomers use a unit of measurement called the Light Year, or the distance that light travels in one year of time. Since light travels at a speed of 300,000 km/second, it can go pretty far in one year. There are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and 365.25 days in a year. Light travels over 9,400,000,000,000 km in one year. That is 9.4 TRILLION km. At light speed, I can shine a flashlight at the moon, and it will arrive 1.2 seconds later. I can shine my flashlight at the Sun, and the light will arrive there 8 minutes later. Sunlight cover the entire 5.9 billion km, or 30 AM distance to Pluto in about 4 hours. But when we talk about the distance to other stars, we look at lightYEARS! The stars in space are incredibly far apart. They may look close to us and to each other against the backdrop of a black night sky, but in reality the distances between them is staggeringly huge. Light requires a journey of 4.3 years to arrive at the closest neighbor star, Proxima Centauri.

For even more distant objects, astronomers have coined the terms Parsecs and Megaparsecs. One Parsec (which will be defined in greater detail later) is 3.26 light years. One Megaparsec is 1 million parsecs, or 3,260,000 light years.

Unit of Far Distance Measurement


1 Meter (m)

39.77 inches ... just a bit longer than an English yard

1 Kilometer (km)

1000 meters ... 0.62 miles

1 Astronomical Unit (AU)

149,600,000 kilometers (the distance from Earth to Sun) ... 93 million miles

1 Light Year (ly)

9.46 x 10^12 km (how far light travels in one year at 300,000 km/sec speed)

1 Parsec (pc)

3.26 light years

1 Megaparsec (Mpc)

1 million parsecs

Measuring the Distance to Stars

Students often wonder how astronomers figure out where these stars are in space without actually going to them. There are 5 very important tools or techniques that are used to determine the distance to stars and galaxies. I have devoted an entire page to these techniques, found at Measuring Star Distances. From the most accurate to the least accurate method of measuring, astronomers utilize techniques of 1) Parallax, 2) Spectroscopic Parallax, 3) Using Cepheid Variable Stars as Standard Candles, 4) Using Type Ia Supernovae as Standard Candles, and 5) Hubble Law. While you are only introduced to the title of these techniques, you will be expected to learn them through your study of this course.

Units of Energy

Most Americans are familiar with temperature measured in units of degrees Fahrenheit. Scientists are far more interested in measuring the energy of molecules than in getting a feel for how hit something is. While the metric measure of temperature is in degrees Celsius, a better system was developed by British scientist Sir Kelvin. Kelvins are units is temperature which measure the kinetic motion of molecules, or their vibrational energy. At a temperature of 0K, there is no atomic kinetic motion ... the molecules essentially stop moving or vibrating. This lowest possible temperature is called Absolute Zero. At a temperature of 273K, water freezes. At 373K water boils. Humans maintain an internal body temperature of 98.6 degrees Fahrenheit, or 37 degrees Celsius, or 310 degrees Kelvin. We measure the energy of stars in terms of Kelvins because this will tell us about the kinetic energy of their layers. For instance, the surface of the Sun has a temperature of 5700K. The Corona, or outer atmosphere of the Sun has a temperature of 1,500,000K. This incredibly hot atmosphere would not feel as hot to the touch as the surface layer even though the kinetic energy of the molecules is much higher. The surface feels hot because the molecules are more closely packed together. The Corona feels cooler although higher in Kinetic energy because the molecules are spread out farther from each other. For the most part, we will refer to temperatures in Kelvins, and occasionally in degrees Celsius.

There are other energy units which deal with forces, but these will be introduces and discussed later.

While you can return to the Introduction to the Starry Sky it is time instead to find out what is up there that you are looking at. In other words, what are those Celestsial Objects, that you see shining against the blackness of the night sky. I can imagine the suspence you must be feeling as you anticipate looking at the celestial objects page, but if you are tiring, you can go back to the Syllabus or the Home page.

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