Astronomical Measurements

Massive suns, billions of years, the vastness of space – astronomy is literally a science of epic proportions. Considering this, the measurements we use for every day practical purposes, such as weighing ourselves or defining distances between one city and the next, are relatively ineffective for astronomical accuracy.

This isn’t always the case though. There are measurements we use daily which also have their place in this all-encompassing science. Here we will take a look at various astronomical measurements for distance, mass, temperature and time.


Astronomical Units

Astronomical unit
The astronomical unit, most commonly abbreviated as AU or au, is a unit of length approximately equal to the distance between the Earth and the Sun. We round this number off to 150 million kilometres (93 million miles) however, that distance isn’t constant, varying as the Earth orbits the Sun. This is because the orbital plane of the Earth is not a perfect circle, but elliptical. Having an elliptical orbit means that the Earth has points in its revolution when it is at maximum distance from the Sun ( called aphelion) and at a minimum distance from the Sun (known as perihelion).

Astronomers use the astronomical unit mainly for defining distances within solar systems. It is also used for scale in stellar systems, such as the size of a comet’s tail, or the size of a protostar’s disc.

Astronomers will not use astronomical units when measuring interstellar distances as it is too small a unit for these great spaces. Instead, scientists will use the parsec or, less commonly, the light year.

Light Years

Unlike what the name suggests, the light year is a measure of distance and not of time. Simply put, it is the distance that light can travel (in a vacuum) in one Julian year, or 365.25 days. Light in a vacuum is incredibly fast – no other known object can travel as fast as light does. Moving at nearly 300 000 kilometres per second, light covers approximately 9.46 trillion kilometres (5.88 trillion miles) in one year. The unit is sufficiently big enough to describe large astronomical distances such as the spaces between stars and even between galaxies.

Stars in the same cluster can be a couple of hundred to thousands of light years apart, whereas galaxies are often separated by millions of light years. The furthest objects from the Milky Way galaxy are billions of light years away.

In spite of the enormity of the light year and the convenience of measuring large distances, most professional astronomers do not use the light year as a standard unit of measurement, preferring the officially recognized unit of parsecs for both interstellar and intergalactic distances. The light year does however remain popular in mainstream science magazines.

The International Astronomy Union (IAU) use the abbreviation ly for light years, whereas other institutions also acknowledge l.y as an abbreviation, as well as language specific abbreviations.

Light years can further be defined in even bigger measurements including the kilo light year (1000 light years), the mega light year (100 000 000 light years), and the giga light year (1 000 000 000 light years).

On the other hand, we can also divide light years into smaller units such as the light second (the Moon is about a light second away from Earth) and light minutes, hours, days and even light months.


To understand the parsec, we first need to know what parallax is. Have you and a friend ever disagreed over the position of the Moon because you were standing in different places in the yard? You may say that the Moon is hanging above a chimney, while your friend swears its hanging above the trees. Parallax is this perceived change in the position of an object when seen from two different places.

Astronomers apply parallax by observing astronomical objects at different times of year. Scientists usually wait half a year between the two observations. Because the Earth’s orbit is exactly known, we can use trigonometry to calculate the distance of the observed object from Earth. This distance is expressed in parsecs. Most of us are familiar with using degrees in trigonometry, but the resultant angles in this method are too small to express clearly in degrees. Scientists therefor use smaller divisions of the degree called arc seconds and arc minutes (or seconds of arc and minutes of arc). One second of arc is equal to 0.000277778 degrees.

One parsec is the distance to an object that has a parallax angle of one arc second. A star with 1/2 arc second of parallax is two parsecs away. A star with 1/3 arc second of parallax is three parsecs away and so on.

The parsec is the preferred unit of distance used by professional astronomers, though the unit is applied mainly for shorter distances within the Milky Way. Multiples of parsecs, including kilo parsecs, mega parsecs, and giga parsecs are used to express distances within and between galaxies.  A parsec is equal to approximately 3.26 light years.


Earth Mass

Calculating Earth MassAn Earth mass (M) is a unit of mass approximately equal to 6 x 1024 Kilograms, which is the approximate mass of the Earth (determining the exact mass of the Earth is a tricky). Astronomers use the Earth mass to reference the mass of other rocky terrestrial planets within the Solar System, as well as planets of a similar structure and composition outside of the Solar System.

The unit for the Earth mass only includes the mass of the Earth, not of the Earth system which would include the Moon. Regardless, including the Moon in the total unit has a very minute effect on the result, as the Moon’s mass is so much less than that of Earth.

The bulk of what makes up Earth’s mass includes mainly iron and oxygen, as well as magnesium, silicon, aluminium and nickel.

Jupiter Mass

Jupiter mass (MJ), also referred to as Jovian mass, is the unit of mass equal to the mass of the planet Jupiter. This is approximately 1.898 x 1027 kilograms. Jupiter is the most massive object in the solar system apart from the Sun itself. The planet is roughly 318 times more massive than the Earth, and to highlight its size even further, it has a mass roughly 2.5 more than all the other major planets combined.

Unlike the Earth mass unit, when we refer to the Jovian mass, we may be talking either about the mass of the planet Jupiter or to refer to the mass of the Jovian system, including the planet’s natural satellites.

Astronomers use the Jovian mass unit to describe planets of a similar size to Jupiter. This includes the Solar System gas giants Saturn, Uranus and Neptune, as well as massive exoplanets which are often called hot Jupiters. Additionally, the Jovian mass may also be used to express the masses of brown dwarfs: celestial objects which are the size of a giant planet or a small star, and which emit infrared radiation.

Solar Mass

Finally, we come to the solar mass (M); a unit of mass which is equal to the mass of our Solar System’s Sun. This value is approximately equal to 2 x 10 30 kilograms. For a better idea of how big this is, 1 solar mass is approximately 332 946 greater than the mass of Earth and 1048 times that of the Jovian Mass!

It was actually the great Scientist Isaac Newton who was the first person to estimate the mass of our Sun. This mass is not constant because of the process of stellar evolution. As a matter of fact, our Sun is becoming increasingly less massive. One such way this happens is when mass is converted to energy through the nuclear reactions that take place inside a star (hydrogen fusing into helium). The converted energy eventually radiates away from the Sun. The Sun also loses mass when high energy protons and electrons are ejected from the Sun’s atmosphere into outer space.

We use the solar mass to express the masses of other stars, but also to define the masses of star clusters, compact celestial objects like neutron stars and black holes, nebulae and galaxies.



You will either be familiar with Celsius (or Centigrade) or Fahrenheit depending on where in the world you live. Celsius is part of the metric system and is used in most parts of the world, whereas the Fahrenheit temperature is used predominantly in the USA. These two scales are ideal for practical daily use, where we require a system that uses reasonable numbers to describe ordinary conditions on Earth. For example, water freezes at 0° C and boils at 100° C. These scales are ideal for expressing normal weather conditions. In fact, Earth’s average temperature is around 15° C (49° F).

For astronomy, these temperature scales are too small. Temperatures in space are often far colder or far hotter than anything we experience on Earth, so we need a more efficient temperature scale. The Kelvin temperature scale is the perfect solution. To convert using the metric system, simply take the temperature in Celsius and add 273.15 to get the degrees in Celsius.

Kelvin does not use any negative numbers, making it especially useful for describing the iciness of a comet or frozen Moon. It is also good for defining the high temperatures of hot stars or supernovae events because the scale’s degrees are “bigger” than that of both Celsius and Fahrenheit. Astronomers use Kelvin for the classification of stars and their place on the Hertzsprung- Russell diagram which plots stars based on, in part, their surface temperatures. The most convenient aspect about using Kelvin in astronomy is that 0° Kelvin is equal to absolute zero: the point at which the fundamental particles of nature have minimal vibrational motion, and the coldest temperature there is in the natural universe.


The astronomical unit for time is the day (D), equal to 86 400 seconds. A Julian year is made up of 365.25 days, the approximate time it takes for Earth to make one full revolution around the Sun. The day can be divided into hours, minutes and seconds; and makes up weeks and months too.

In astronomy, there are two different types of “days”.

Sidereal Time

Sidereal time

Sidereal time is a system used by astronomers not only used for timekeeping, but also to locate celestial objects. Sidereal time is based on the Earth’s rate of rotation measured relative to the fixed stars. All this essentially means is that sidereal time is the time it takes for a specific star to be in the exact same position in the sky over two successive nights. This is similar to how we can locate the Sun using the time kept by a sundial.

A sidereal day is equal to approximately 23 hours, 56 minutes, and 4.0905 seconds. This is about four minutes shorter than a solar day. Due to the difference in time between the sidereal day and the solar day, after a year has passed, there will have been one more sidereal days in that year than solar days. Earth makes one rotation around its axis in a sidereal day; during which time it moves about 1° in its orbit around the Sun. But even after one sidereal day has already passed, the Earth still needs to rotate a bit more before the Sun reaches the same position as the previous day, therefore being longer than a sidereal day.

Like most things in an ever changing cosmos, sidereal time does not remain constant. Earth’s rotation is not simple. Over a long enough period of time, you would see the Earth spinning about its axis in a way similar to a spinning top, making an almost circular motion about a point. It takes about 25,800 years to make a complete rotation around this point. This phenomenon is called the precession of the equinoxes. Because of this precession, the stars do not remain fixed in their positions as seen from Earth. Given this long period of time, Polaris (the North Star) will actually no longer be the North Star!

Solar Time

Solar TimeSolar time is a different way of measuring a day, but it is very similar to how we measure sidereal time. Imagine the Sun casting a shadow of a tall pole. At some during the day, the shadow will point exactly north or South, and if the Sun is overhead, the shadow will disappear. If we wait a day, the Sun will have seemed to cover a 360° arc around the sky, coming to the same point and casting the same shadow about 24 hours later. The Sun covers about 15° across the arc of the sky for every hour.

Equation of time

Again, we see that because we are not dealing with perfect unchanging measurements in astronomy, this type of time keeping isn’t constant. There are certain times of the year when the sky takes less time to cross the 360° arc than other points in the year when it may take slightly longer. This is because the Earth does not have a perfectly circular orbit about the Sun, and is sometimes further or closer to the star.

To make up for this and to keep time more accurately, we have to follow an imaginary Sun called a “mean Sun” which moves at a constant rate along the arc and matches the real Sun’s average rate over the year. Time keeping using a mean Sun is called “mean solar time”. The real sun is also referred to as the apparent Sun, and time kept following the motions of the apparent Sun is known as apparent solar time.


Astronomy is so mind-bogglingly vast that it is actually quite wonderful how well we can express the universe’s dimensions and state of being. Not all our methods fit perfectly into the constantly changing cosmos, but mathematics never fails at expressing these dimensions ever more accurately and eloquently.

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