Turn the sky. Find the match.
Move the dial and watch the Moon change. Then play a round: set the sky to match the request and open the guide behind it. No sign-up, no timer, no daily limit.
Five instruments, five cycles
Each category has its own instrument in the game: a Moon Dial, an Alignment Bench, an Orbit Dial, a Clock Dial and a Rise & Set Dial.
- Moon Phases and MotionWhy the Moon changes shape, how long a cycle takes and what you see from each hemisphere.
- EclipsesSolar and lunar eclipses: how the Sun, Earth and Moon line up, and why it does not happen every month.
- Seasons and the Sun’s PathSolstices, equinoxes, the tilt of Earth’s axis and how the length of the day changes.
- Calendars and ClocksSolar and lunar calendars, leap years, time zones and the time the Sun tells.
- Watching the SkyWhen the Moon and Sun rise and set, and how to watch the sky and keep a log of what you see.
Start with these guides
Each guide unlocks when you play its round, or with one click if you would rather just read. Winning a round saves the entry in your Sky Logbook. See all 45 guides.
- Full MoonThe full Moon is the Moon facing the Sun's opposite side of the sky.
- Total Solar EclipseA total solar eclipse is the Moon's darkest shadow, the umbra, landing on Earth.
- Summer SolsticeAt the June solstice, Earth's axis leans the Northern Hemisphere as far toward the Sun as it ever goes, so the noon Sun stands highest there.
- Leap YearsYears divisible by 4 are leap years, except century years that are not divisible by 400.
- TwilightTwilight is the glow before sunrise and after sunset.
- First Quarter MoonHalf of the Moon's disk is bright, yet the name says quarter.
How a round works
- 1
Read the request
It tells you what the sky should look like, such as a Full Moon, a total solar eclipse or the date of a solstice.
- 2
Turn the dial
Use the slider, the arrow buttons or the keyboard. The picture changes as you go.
- 3
Lock it in
A match earns a medal and a logbook card. A miss shows where the sky really was.
Words of the sky
Six terms the guides use. The definitions come from the U.S. Naval Observatory.
- Age
- The time in days counted from the time of New Moon.
- Lunation
- One complete cycle of phases, 29.5 days on average.
- Waxing
- The lit part of the Moon’s disk is getting bigger each night.
- Waning
- The lit part of the Moon’s disk is getting smaller each night.
- Crescent
- The Moon looks partly lit, but less than one-half.
- Gibbous
- The Moon looks more than one-half lit, but not full.
How we write these guides
- We open every source ourselves and print the date we opened it on the page.
- Each entry is checked against at least two official sources. When they differ, the guide says so.
- The drawings are our own and are computed from the numbers in the sources.
- We do not invent quotes, numbers, experts or reviews.
- There are no ads and we set no cookies. Your logbook stays in your own browser. Cloudflare, our network provider, adds a visit counter to our pages, and the Privacy page explains it.