The sky tonight

How the page works

Start typing a town or city and pick it from the list. The page finds the night you are in, or the coming one if the Sun is still up: the moment of sunset, the next sunrise, and the stretch in between when the Sun is at least twelve degrees below the horizon and the sky is properly dark. It then works out where the Moon and the five bright planets stand every ten minutes of that night, draws their paths, and writes a line for each one underneath. If a birth chart is saved on this device and no place has been chosen, the birth place stands in, and the note under the field says so.

How to read the drawing

The drawing is the whole sky unrolled into a strip: the horizon along the bottom, overhead along the top, and every direction across it, with south in the middle for anywhere north of the equator and north in the middle south of it, because outside the tropics that is the side of the sky the planets keep to. Each coloured line is where one planet travels between sunset and sunrise, with a dot on every hour, and the slider stands you at any minute of the night.

Height is given in degrees above the horizon. A fist held out at arm’s length covers about ten of them, so a planet thirty degrees up is three fists above the ground. Anything under ten degrees is easily lost behind buildings and trees.

Reading a line, with an example

Each line gives when the planet rises or sets, the highest it gets once the sky is dark, and how bright it is. Take a line that reads “rises 21:40, up at sunrise” beside “Jupiter, highest at 48° in the south at 01:30” and “brighter than any star”. Jupiter clears the horizon at 21:40, climbs through the night, and is best placed around half past one, a little over halfway up the southern sky, about five fists above the ground, and brighter than any star near it. The highest point only counts once the Sun is six degrees down, when the brighter planets begin to show; a planet that sets before then is listed as only up in the twilight, low and gone before the sky is dark. For the Moon the line gives how much of its face is lit instead of a brightness.

Why only these six

The Moon, Mercury, Venus, Mars, Jupiter and Saturn are the ones you can see with nothing but your eyes. Uranus is at the very edge of eyesight under a perfectly dark sky, and Neptune and Pluto need a telescope, so they are left off rather than listed as something to look for. Mercury never strays far from the Sun, which is why it only ever shows low in the twilight, just after sunset or just before sunrise. The weeks when it is furthest from the Sun, and so easiest to catch, are on Mercury cycle, and which planets are morning or evening objects across the whole year is on Visibility.

Twilight, city lights and the horizon

The sky darkens in stages. While the Sun is less than six degrees down, the slider calls it twilight, and Venus and Jupiter already show; between six and twelve degrees it is nearly dark; beyond twelve the page counts the sky as dark. Around midsummer far enough north, in Scotland or Scandinavia, the Sun never gets twelve degrees down, and the page says so rather than inventing a dark hour.

Street lighting hides faint stars long before it hides planets. Venus, Jupiter and Mars near its best are easy from a city centre, and Saturn usually is too. A bright Moon washes out the fainter stars but not the planets; its phase tonight is on The Moon, and the showers that do need a dark, moonless sky are on Meteor showers. What spoils a view of a planet more often is the horizon itself: roofs, trees and haze in the direction it sits.

Times and places

Times are given by this device’s clock. If you look up a city in another time zone, for a trip or for a friend, the rise and set times are still in your own zone rather than local time there. Inside the polar circles there are weeks when the Sun does not set, and on those dates the page says there is no night to draw. When two planets come close together in the sky, the dates are on Conjunctions.

How bright

Brightness comes from the planet’s magnitude, the astronomer’s scale on which a lower number is brighter. Venus at its brightest is outshone only by the Sun and the Moon; Jupiter usually beats every star; Mars swings from dazzling to ordinary over two years as its distance from the Earth changes.

The positions are worked out on this device for the place you give, to within a minute or so, and allow for the way the air bends light near the horizon. The sky is only the sky: nothing here says what a planet being up means for anybody.