What it is
Astronomy is the study of what is actually overhead. Every view of the sky mixes ages: the Moon as it was a second ago, the Sun eight minutes ago, distant galaxies as they were billions of years ago.
Modern astronomy works from space as much as from the ground. ESA’s Gaia has charted the positions and motions of more than a billion stars in our galaxy, and NASA’s James Webb Space Telescope looks in infrared at the first galaxies.
The planets, to scale in distance and time
Shows how far apart the planets really are and how long each year lasts, using NASA’s figures.
| Planet | From the Sun | Diameter | One year | Moons |
|---|---|---|---|---|
| Mercury | 57.9 M km | 4,879 km | 88 days | 0 |
| Venus | 108.2 M km | 12,104 km | 224.7 days | 0 |
| Earth | 149.6 M km | 12,756 km | 365.2 days | 1 |
| Mars | 228 M km | 6,792 km | 687 days | 2 |
| Jupiter | 778.5 M km | 142,984 km | 4,331 days | 95 |
| Saturn | 1,432 M km | 120,536 km | 10,747 days | 274 |
| Uranus | 2,867 M km | 51,118 km | 30,589 days | 28 |
| Neptune | 4,515 M km | 49,528 km | 59,800 days | 16 |
Distances use a logarithmic scale so all eight fit; on a true scale Neptune would sit 78 times farther out than Mercury. Planet sizes are exaggerated; orbital speeds are sped up but keep their order.
Source: NASA Goddard (NSSDCA)
Ideas to carry
- 1
Lookback time
The farther away something is, the older the light you see from it.
- 2
The scale problem
The planets are tiny and far apart. Any picture that shows both their size and their distance is lying about one of them.
- 3
The sky moves
Earth spins and orbits, so the sky is a slow clock and calendar.
Established What we know
- Neptune orbits about 4.5 billion km from the Sun and takes about 59,800 days, roughly 164 years, to go round once.[1]
- ESA’s Gaia mission has mapped the positions and motions of more than a billion stars in the Milky Way.[2]
- The James Webb Space Telescope, launched in December 2021, observes in infrared to see the earliest galaxies.[3]
Interpretation The older lens · History of science
Astronomy and astrology were one discipline until the 17th century; Kepler still cast horoscopes while finding his laws.
Reading
- c. 150AlmagestPtolemy
- 1543On the RevolutionsNicolaus Copernicus
- 1610Sidereus NunciusGalileo Galilei
You: stargazing
The stars: this light is older than your entire family tree
Connected subjects, projects and writing
- Space explorationSubject · Frontier
Established Spacecraft and space telescopes are how modern astronomy sees.
- Black holesSubject · Cosmos
Regions where gravity is so strong that nothing, not even light, can escape: from theory to photographs.
- RelativitySubject · Cosmos
Einstein’s account of space, time and gravity, and why your phone’s map needs it to work.
Sources
- Planetary Fact Sheet — Metric NASA Goddard (NSSDCA). Checked 5 Nov 2026.
- Gaia European Space Agency. Checked 5 Nov 2026.
- James Webb Space Telescope NASA Science. Checked 5 Nov 2026.