Definitions, history and context of units of measurement, with support content for students.
An astronomical unit equals exactly 149,597,870.7 kilometers. It’s the average distance between Earth and the Sun, and astronomers use it as a measuring stick when kilometers fall short for expressing distances in the solar system. If you need the exact figure and where it comes from, here’s everything explained without detours. What is an astronomical…
The closest star to Earth, not counting the Sun, is called Proxima Centauri and it’s 4.24 light-years away. That means the light leaving it takes 4 years, 2 months and a few days to reach us. If that star went out today, we’d still see it shining for more than four years. What a light-year…
The Sun has a diameter of 1,392,700 kilometers. That number, written like that, doesn’t say much. But if I tell you that 1.3 million Earth-sized planets would fit inside the Sun, the scale starts to make sense. In this article you’ll see exactly how the size of the Sun compares to Earth’s, with exact figures…
The Sun has a mass of 1.989 × 10³⁰ kilograms. That figure is so large that in astrophysics it stopped making sense to use kilograms, so a unit of its own was created: the solar mass (M☉). When we talk about black holes, this unit becomes essential, because their masses range from a few times…
The distances between planets aren’t fixed: each planet orbits the Sun at its own speed, so the separation between two worlds constantly changes. Here are the key figures in kilometers and miles, with average distances to the Sun and the minimum and maximum possible distances between neighboring planets. Why the distance between planets isn’t a…
The Milky Way measures between 100,000 and 200,000 light-years in diameter, depending on where you consider its stellar disk to end. The figure most commonly cited in recent scientific publications is 100,000 light-years for the visible disk, although studies that include the diffuse stellar halo extend that measurement up to 200,000 light-years. Let’s break down…
Earth weighs 5,972 trillion tons, written out in full: 5,972,000,000,000,000,000,000 tons. It sounds like a made-up number, but it’s the real mass of our planet, calculated from its gravity and volume. Compared to the rest of the solar system, Earth is a marble: Jupiter weighs 318 times more, and the Sun contains 99.86% of all…