Printable Cheat Sheets
The planets at a glance
| Planet | Type | Diameter (km) | Distance from Sun (AU) | Year length |
|---|---|---|---|---|
| Mercury | Rocky | 4,879 | 0.39 | 88 days |
| Venus | Rocky | 12,104 | 0.72 | 225 days |
| Earth | Rocky | 12,742 | 1.00 | 365.25 days |
| Mars | Rocky | 6,779 | 1.52 | 687 days |
| Jupiter | Gas giant | 139,820 | 5.2 | 11.9 years |
| Saturn | Gas giant | 116,460 | 9.5 | 29.4 years |
| Uranus | Ice giant | 50,724 | 19.2 | 84 years |
| Neptune | Ice giant | 49,244 | 30.1 | 165 years |
1 AU (astronomical unit) is the average Earth–Sun distance, about 150 million km. Values are rounded.
Life of a star
| Step | Stars like the Sun | Much more massive stars |
|---|---|---|
| 1. Birth | A cloud of gas and dust collapses in a nebula and a protostar forms. | |
| 2. Main sequence | Fuses hydrogen to helium for about 10 billion years. | Burns fuel much faster and lives only millions of years. |
| 3. Later life | Swells into a red giant. | Swells into a red supergiant. |
| 4. Death | Sheds its outer layers as a planetary nebula. | Explodes as a supernova. |
| 5. Remnant | A white dwarf. | A neutron star or a black hole. |
Moon phases in order
New Moon → waxing crescent → first quarter → waxing gibbous → full Moon → waning gibbous → last quarter → waning crescent. The cycle takes about 29.5 days. “Waxing” means the lit part is growing. The Moon’s lit half always faces the Sun; the phase depends on how much of that half we see. A solar eclipse happens at new Moon and a lunar eclipse at full Moon, but only when the Sun, Earth and Moon line up exactly.
How long light takes
| To the Moon | about 1.3 seconds |
| To the Sun | about 8.3 minutes |
| To Neptune | about 4 hours |
| To Proxima Centauri | 4.2 years |
| Across the Milky Way | roughly 100,000 years |
A light-year is a distance (about 9.46 trillion km), not a time.
Constellations by season (northern hemisphere)
| Winter evenings | Orion, Taurus, Gemini, Canis Major, Auriga |
| Spring evenings | Leo, Virgo, Boötes, Ursa Major |
| Summer evenings | Cygnus, Lyra, Aquila, Scorpius, Sagittarius |
| Autumn evenings | Pegasus, Andromeda, Cassiopeia, Perseus |
Always visible from mid-northern latitudes: Ursa Major, Ursa Minor, Cassiopeia, Cepheus and Draco. Find them on the star map.
Size and brightness words
- Apparent magnitude: how bright a star looks from Earth. Smaller (or negative) numbers are brighter. 5 steps equal a factor of 100.
- Light-year: distance light travels in a year.
- Parsec: about 3.26 light-years.
- Event horizon: the boundary around a black hole beyond which nothing can escape.