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Understanding Astronomical Distances and Measurement Methods

Class 7
Jun 10, 2023


A network of NASA satellites known as the Tracking and Data Relay Satellite System is used by Hubble (TDRSS). Hubble can communicate with the TDRSS satellites at a variety of locations around the sky almost continuously. The data from Hubble is periodically sent through TDRSS to TDRSS’s ground terminal at White Sands, New Mexico. The data is then forwarded to Goddard by landline to be checked for accuracy and completeness.

To process, analyze, and store the data, Goddard transmits it over landlines to the Space Telescope Science Institute in Baltimore, Maryland. This raw data is converted there into scientific data, such as wavelength and brightness, and finally into the best possible images taken by the Hubble telescope.

Astronomers have created units to measure the distance of celestial objects they observe because they are so far away.




Astronomical Numbers:

Do you ever consider how far away the stars are when you look at the night sky?

Proxima Centauri, the nearest star, is 42,000,000,000,000 km distant. Many zeros are needed to write out such an enormous distance of 42 trillion.


Scientific Notation:

A mathematical abbreviation for writing extremely large (or extremely small) numbers is called scientific notation. Numbers are expressed using this approach as a value between 1 and 10, multiplied by a power of 10.


Scientific notation

For example, 42 trillion km can be expressed as 4.2×1013 km.

There is a 150,000,000 km (approx.) distance between the Earth and the Sun. We use scientific notation to express this value.

Steps to Write a Number Using Scientific Notation:

Step-1. Move the decimal point until you obtain a number between 1 and 10. Keep track of how many times you move the decimal.

Step-2. Remove all the zeros from the new number and write them down.

Step-3. Add an (x 10) to the end of the number.

Step-4. Express your number of decimal moves as a power of 10. (The exponent).




The exponent will be positive if you slide the decimal one place to the left. The exponent would be negative if you shifted the decimal to the right.

Astronomical Distances:

Astronomical Unit:

The average distance between the Earth and the Sun is called 1 Astronomical Unit (A.U.).

Light Years

light year

The name light-years (Ly) might imply time, but this unit does not measure time. The distances in space are measured in terms of light years. The distance that light covers in one year (9.46 x 1012km) in space is measured as a light year.


Another measurement of distance employed by astronomers is the parsec (pc). A parsec is a measurement devised from geometry and trigonometry to express the position and separation of objects from Earth in space.


There are 32.6 light years in ten parsecs.

3.26 light-years = 1 parsec

If the brightness of a star is 10 parsecs from the Earth, then it is known as a star’s luminosity.

Determining Distances of Closer Objects in the Universe:

Measuring the Distance of Closer Stars:

Parallax Method:

When a star is less than 1,000 light years away from Earth, astronomers utilize a technique called parallax to calculate its distance.

Parallax Method

Hold one finger around six inches from your nose to demonstrate parallax. Close your left eye and focus your right eye on your finger. Then, close your right eye and focus your left eye on your finger. Your finger seems to be moving because your eyes seem to be moving in separate directions.


The same is true of the stars in the sky. Over the course of a year, the nearby stars appear to move in the sky as Earth revolves around the Sun. While distant stars remain stationary in fixed positions in space.

Distance of a star

How to Measure Distance Using Parallax:

Astronomers use parallax to determine a star’s position in the sky about other stars that are too far away to move. They then observe the star six months later, when Earth is on the other side of the Sun, to estimate how its position has changed concerning distant stars.

stars in the sky

They can calculate the distance between the star and Earth using geometry.

Limitation of Parallax Method:

Parallax only works for closer stars.

As the distance from Earth increases, the change in a star’s angle becomes less measurable or very small. Therefore, parallax only works for stars that are relatively close to Earth.

parallax angle

This can be shown by continuing to look at the finger that is in front of your nose as you did previously. Try moving your finger away from your nose this time while keeping each eye focused on it. The movement appears to get smaller as it gets further away until there is no longer any movement seen.


Light Years and Time:

The age of an object’s light when it reaches Earth equals the object’s distance from Earth in light years. For instance, the Proxima Centauri light we saw departed that star 4.2 years ago.


Similarly, when astronomers study the Andromeda galaxy, they are looking back in time 2.5 million years.


Time as a Tool of Astronomy:

The more back in time we are viewing, the further away an object is. Astronomers now highly depend on this fact to create their cosmology models. Astronomers have created hypotheses about how stars are created, live, and die by comparing stars that are nearby with stars that are very far away.

age of universe

Solving Problems:

How long does it take for radio waves to travel from the Moon to Earth?

Moon on the earth


The distance between the Earth and the Moon is 384,400 km.

The speed of light is 300,000 km per second.

We know, speed = distance / time.

Time = 384,400 km / 300,000 km/sec = 1.28 s

Thus, the radio waves travel from the Moon to the Earth in 1.28 seconds.

Astronomical Distances


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