Sky Map
Where the sun, the moon and the planets are, for any place and any date, computed in your browser from orbital elements. This page runs on the same arithmetic an almanac is using without fecthing any external data.
The sky now
Looking up from your place at the chosen moment. North is at the top, the outer circle is the horizon, the centre is straight overhead. The thin arc is the sun's path across this day, the dashed arc the moon's.
Right now
| Body | Altitude | Azimuth | Rises | Transits | Sets | Distance |
|---|
Altitude is degrees above the horizon, azimuth degrees clockwise from north. Distances are in astronomical units, except the moon, in Earth radii.
The day
Darkness by degree, hour by hour. Civil twilight is when you can still read outdoors, nautical when the horizon is still visible at sea, astronomical when the last glow leaves the sky.
The moon
Next phases
The year
Twilight calendar
Every day of the year across the page, the clock down the side. The white band is daylight, the greys are the three twilights, and the dark core is true night. The vertical line marks the chosen date.
Analemma
The sun's position at the same clock time every day for a year. The figure of eight comes from the tilt of the Earth and the shape of its orbit, which is also why noon by the clock and noon by the sun drift apart by up to a quarter of an hour.
How it works
To figure out where planets are right now, we start with a map of their general paths called orbital elements. First we calculate where a planet would be if it moved at a perfectly steady speed in a perfect circle. We find this average spot by taking its starting point and adding its daily speed multiplied by the time that has passed:
But planets actually travel in squashed circles called ellipses speeding up and slowing down. To find the real position we have to adjust our perfectly circular guess using a famous mathematics trick that slowly guesses closer and closer to the right answer based on this rule:
Once we have that we place the sun in the center and map everything out on a flat grid. Since we watch the sky from Earth and not the sun we add the position of Earth to shift our perspective. Then we tilt the whole grid to match the natural tilt of Earth using a rotation rule like this:
Finally we need to know what you specifically can see from your backyard. We take your exact location on the globe and the current time to twist the grid one last time. This tells us how high the planet is in the sky and what compass direction to look toward using round globe mathematics:
The moon is so close to us that we have to add a bunch of extra tweaks to account for the gravitational pull of the sun wobbling it around plus a fix depending on where you stand on the surface of Earth.
To find out when the sun or moon will rise and set, the code simply steps through the day two minutes at a time and watches for the exact moment the object crosses the horizon. For the sun we actually look just a tiny bit below the edge to account for the atmosphere bending the light and the actual size of the sun. For the moon we adjust for how close it is. The twilight phases are just the sun hitting deeper and deeper specific angles below the horizon.
Overall this mathematics is extremely accurate, and usually gets the rise and set times correct within a minute.
Oh, and just remember that time zones are fixed. So if you are looking at a date during daylight saving time, the clock might look one hour off unless you adjust the offset yourself.