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How Big Is the Moon?

Using Slooh’s Online Telescope and integrated Quest learning activities, you can capture your own images of the Moon and other celestial objects while learning more about their magnificent sizes. How Big? is one of 60+ curriculum-aligned STEM Quest learning activities on Slooh for students 4th grade to college.

Slooh’s Online Telescope:


How Big Is the Moon?

The Moon appears much larger than any other object in the night sky, but this is only because it is the closest celestial object to the Earth. In reality, the Moon has a diameter of 3,475 kilometers, which is about a quarter of the size of the Earth!

Instead of reporting the physical sizes of celestial object in standard units of measurement such as kilometers, we prefer to think about their sizes in more relatable terms, making them easier to understand. While the Earth’s diameter is a good starting point since it is relatively familiar, we can also compare the Moon to objects that are even more well known to us to gain a better understanding of its size. As such, the Moon’s diameter is slightly less than the width of the USA measured from the east to west coast (4,500 km). Its diameter is also very similar to the width of Australia, which measures 3,606 km!

The Moon in comparison to the USA. Image by Backyard Stargazers

There are over 200 other moons in the Solar System! The Earth only has one moon, but Saturn has a total of 82, while Mercury and Venus have none. Our Moon is the largest relative to the size of its respective planet, which allows it to stabilize the Earth’s seasons. However, it is not the largest overall. The table below shows the sizes of some of the largest and smallest moons that have been discovered in our Solar System.



Diameter (km)




















More About Slooh's How Big? Quest

You've seen many amazing photos of celestial objects, perhaps without knowing exactly what you are looking at, and over time you may have become a bit desensitized as to their majesty. On this quest, you will attempt to regain your perspective of these glorious celestial wonders.

Learning Objectives

By the end of this Quest, students will be able to answer the following questions:

  • What are the three domains of space?

  • What are the three standard units of measurement for each domain?

  • How do you convert the size of an object from unit to another?

Vocabulary Words

Standards Addressed


​RST.9-10.1, RST.9-10.2, RST.9-10.3, RST.9-10.4, RST.9-10.5, RST.9-10.10, WHST.9-10.2.E


HSN.Q.A.1, HSN.Q.A.2​

Related Slooh Quests

  1. How Far?

  2. The Mystery of the Changing Moon

  3. The Moon - Lunar Features

About Slooh's Astronomy NGSS Aligned Learning Activities

Slooh’s Online Telescope is a learning platform designed to support any educator in teaching astronomy to meet NGSS requirements by collecting and analyzing real-world phenomena. No previous experience with telescopes is necessary to quickly learn how to use Slooh to explore space with your students.

You can join today to access Slooh's Online Telescope and all 60+ Quest learning activities if you are able to make astronomy a core subject of study for the semester or year. If you only have a few weeks to study astronomy, we also have a curriculum designed to fit your busy academic schedule and budgetary limitations. To learn more about our offers, click here.


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