How this sky works
The sky behind every page runs on the clock in Vietnam (GMT+7). The timing is real: sunrise and sunset, twilight, the turning of the night sky, the moon and the meteor showers. Most of the stars are scattered to look like a sky, but the constellations around Polaris, the Milky Way and the Andromeda galaxy are where the real ones are.
day and night
Sunrise and sunset come from where the sun is on today's date, worked out for a point in central Vietnam, to within about a minute. Twilight starts about 70 minutes before sunrise and lasts until about 70 minutes after sunset. As the sky darkens the stars come out brightest first, and at dawn they go out the other way round, faintest first. The page turns from the day chart to the night sky about ten minutes after sunset, and back about ten minutes before sunrise.
the turning sky
Like the real one, the night sky turns around Polaris, up and to the right, by a little over 15° an hour: once every 23 hours 56 minutes. The constellations, among them the Little Dipper, whose handle ends at Polaris, are placed star by star from their real positions and the sidereal time, as you would see them facing north, only scaled to fit the screen. Their colors come from each star's measured color and stay pale, as they look to the eye, fading to white on the faintest. A planetarium such as Stellarium, set to the same time, shows the same picture. Drag the time above to watch them turn. By day the stars are still up there, hidden by sunlight, so the background becomes a star chart of the same sky, turning with it like a planisphere.
the moon
The phase comes from where the moon stands against the sun, using a short form of Jean Meeus' lunar formulas: good to about half an hour on the times of new and full moon. The small moon next to the clock shows the same thing.
The moon itself isn't drawn: it never comes within about 60° of Polaris, well outside the constellations shown here. Its light is. While the real moon is up, the faintest stars, the Milky Way and the Andromeda galaxy fade, the more the fuller and higher the moon is. A moonless night has the most stars.
meteor showers
Around the peak nights of the year's big showers, shooting stars cross the night sky every few seconds instead of every quarter of a minute or so. That is far more than a real sky: even the best showers give a meteor or two a minute, far from city lights.
- Quadrantids3 Jan
- Lyrids22 Apr
- Eta Aquariids6 May
- Perseids12 Aug
- Orionids21 Oct
- Leonids17 Nov
- Geminids14 Dec
small details
When the page turns between day and night, the words fade out, the sky changes color with nothing on it, and only then do the words fade back in. Day and night use opposite colors for text, so a cross-fade would pass through a moment where the words are the same color as the sky.
The stars sit in three layers. When you scroll, near stars drift faster than far ones, while the day chart moves as one flat sheet of paper; moving the pointer shifts the layers a little too. All of this motion, the twinkling and the shooting stars stop if your system asks for reduced motion.
The sky keeps turning behind the words, so by day the chart's ink fades there and the names of things step aside, while its grid runs on unbroken.
things to try
- Click the night sky to send a shooting star.
- Move the pointer near a constellation to see its name.
- Pick day, night or auto at the bottom of any page.