The Sky You Were Born Under

You know the date without thinking and the place without looking. The minute is the one that sends you digging through a drawer, and it's worth the digging, because the sky moves faster than it seems.

Why does the minute matter?

Because the whole sky turns about one degree every four minutes.

Meeus, ch. 12One turn against the stars takes 23 h 56 m 4 s: 15.04° an hour.

An hour moves every star 15°. That's enough to lift a constellation clear of the horizon or to drop another one below it, so the same evening at seven and at ten gives you two different pictures.

For exampleOrion from London in late January. It crosses due south at about half past nine in the evening, high and upright; by two in the morning it's low in the west-southwest and Rigel (β Ori) is about to set.

None of that needs you to know a single constellation. Bring the minute and the chart does the rest.

Where do I find the time?

Usually somewhere it was written down on the day.

For a birth, try the hospital's records, a baby book, the card that went out to the family, or a parent's memory. Some birth certificates record the time and many don't, so it's worth asking whoever was in the room.

For a wedding or the first night in a new home, look at your photos. Your phone stamped every picture with the minute it was taken, and most photo apps show it when you open a picture's details. A camera's clock is only as right as whoever set it, so check one of its pictures against a phone's before you trust it. The time on the invitation is when the ceremony was meant to start; the photographer's files say when it did.

For the night you met, the first message you sent each other is often the closest thing to a timestamp you'll find. Scroll back far enough and the minute is printed beside it.

What if I don't know the time?

Then the minute is yours to choose, and any minute you choose is a real sky.

The studio needs a minute to draw anything, and it won't pick one for you. A few ways people settle it:

• The family story. "Just after midnight" or "around teatime" is a real memory. Set the minute it points to.
• Sunset that day, if the evening is what you remember. The chart on the front page shows when the Sun went down for any date and place you set.
• A round hour, chosen and known to be chosen: nine in the evening, or midnight.

If your minute falls in daylight and you choose the As Seen ground, you'll get the daylight sky: blue, the Sun exactly where it stood, the Moon if it was up, and few or no stars, because that's what was there. It's a lovely print in its own right, and an honest one.

Which place should I use?

The town is close enough. The street doesn't change the sky.

Latitude1° ≈ 111 km (69 miles). Move that far north or south and every star shifts 1°.

You'd have to travel about a hundred kilometres north or south to shift the stars by two Moon-widths. Across a city the difference is too small to see on any print, so set the town, or the hospital, the chapel or the beach if the exact spot matters to you.

Set the time as the clocks read it that day, summer time and all. On the chart on the front page, a city picked from the list carries its own time zone, while coordinates you type are read in your device's zone. So if you live in Chicago and you're setting a London sky, pick London from the list.

What do I choose after that?

How it looks: the view, the ground and the colour of the stars.

That part is all yours. The view is the shape of the print, from the round Dome to the wide Panorama. The ground is As Seen, the sky's own colour at your minute, or a solid colour from Black and White to eight night tones. The stars come in True Colour, each one its own catalogue colour, or in Gold or Silver ink. About the Stars explains each choice, with pictures.

When you have your three, the studio is ready for them, and you'll see the sky draw itself as you set them. If you're wondering how big to go, What Size Star Map Fits Your Wall? walks through the walls people usually have in mind.