New in 1.2

The sky as it looks, tonight and in 357 BC

Version 1.0 put the catalogues on the chart. Version 1.2 puts the air, the Moon's face and the centuries around them. Seven chapters, each with the picture Sky42 drew for it.

01The simulated sky 02The Moon and what it covers 03Five thousand years 04Double and variable stars 05Eclipses and transits 06Your photographs on the sky 07Deep-sky photographs

01 Simulator

A sky with a horizon, a sunset and a twilight in it

The Sky brightness button now has two modes. Atlas is the chart you knew: one colour for the whole sky. Simulator paints the sky as the air does — lighter towards the horizon, a glow round the Sun, the orange band of sunset under the blue, and the Earth's shadow rising in the east once the Sun is down.

The air also takes its share of what shines through it. Stars low in the sky fade and turn orange, and the faint ones go; the Sun and the Moon turn yellow and then orange as they set. The Milky Way, now filled with a faint glow inside its outlines, fades under the Moon or a city sky and is gone by twilight, as the real one is.

The simulated sky at sunset from Venice: the Sun on a straight horizon in an orange glow, fading through pale pink to blue higher up — Sunset from Venice on 7 October 2026, looking west-southwest: the Sun on the horizon, the glow round it, and the blue coming back higher up.

02 The Moon

A photographed Moon, and the planets that go behind it

The Moon is drawn from a photograph of its ground, turned the way it really faces you: the seas, the highlands and the rays of Tycho are where they are, and the seas show in the earthshine of a crescent too. Its panel gives the libration, which tells which edge is showing a little of the far side tonight.

When the Moon passes in front of a planet or a star, that is what you see. The planet fades as the edge covers it, is gone while it is behind, and comes back at the other edge. The Almanac lists these occultations as seen from your place — when each begins and ends, at the dark or the bright edge, and how high the Moon stands.

A crescent Moon with its night side lit by earthshine, and Saturn with its moon Titan just off the dark edge — The same evening as the picture at the top, 18:36 in Padova: two minutes before Saturn, with Titan beside it, goes behind the dark edge of the Moon.

03 Ancient skies

Five thousand years, and an evening Aristotle wrote down

The date field now goes back to 3000 BC. With JPL's DE441 from the Catalogue Manager, the Sun, the Moon and the planets go there too, with their eclipses, conjunctions and occultations in the Almanac. The slow swing of the Earth's axis and the slowing of its rotation follow models made for millennia, and dates before 1582 are in the Julian calendar, as history books write them.

In On the Heavens Aristotle says he saw the half Moon pass in front of Mars, which vanished at the dark edge and came out at the bright one. Set Sky42 to Athens on 4 May 357 BC, the date historians give for it, and it is there: a Moon half a day short of first quarter, Mars gone at the dark edge at 19:53 and back at the bright one at 21:10.

Those times say how far they may be off. Nobody timed the Earth's rotation in 357 BC, so Sky42 writes 19:53 ±6 min — and an hour it cannot vouch for to the minute is not printed as if it could.

3000 BCwhere the Sun, Moon and planets now begin

±6 minstated beside every time in Aristotle's century

The Sky42 window set to 4 May 357 BC in Athens: the half Moon with Mars at its dark edge, and the date field reading minus 0356, 357 BC — Athens, 4 May 357 BC, a few minutes before Mars goes behind the dark edge. The date field reads −0356, the year as astronomers count it, with 357 BC beside it.

The Sky42 almanac for Athens in May 357 BC, listing Mars disappearing behind the Moon at 19:53 plus or minus 6 minutes and reappearing at 21:10 — The Almanac for that month: Mars disappears at the dark limb and reappears at the bright one, every time with its ±6 minutes.

04 Double & variable stars

Where the companion is tonight, and when the star dims next

Select a double star — or a bright one such as Castor — and Orbit… draws the companion's path round the primary, where it is on the date of the sky, and its position angle and separation for the years ahead. The orbits are the published ones, and Update keeps them current.

For a variable, Light curve… shows the curve the Hipparcos satellite measured, where the star is in its cycle tonight, and the next minima or maxima of the stars that keep time — with a plain warning that the real times can run early or late, because periods change.

3,459orbits of visual binaries

12,500variables with a measured light curve

The Sky42 orbit window for the double star STF 1937 AB: an ellipse with the companion marked, and a table of position angle and separation for coming years — η Coronae Borealis, a 41.6-year orbit: the companion in October 2026, and where it will be each year after.

The Sky42 light curve window for Algol: the folded curve with its deep minimum, a line at tonight's phase, and a list of the next minima — Algol, from 57 Hipparcos measures: the dashed line is tonight, and the list gives its next minima.

05 Eclipses & transits

Eclipses that darken the sky, and a Moon that turns red

The Almanac lists the eclipses of the Moon and of the Sun and the transits of Mercury and Venus, with the time of every stage, how deep each one goes, and how high the Moon or the Sun stands where you are. Double-click one and the sky goes there and stays locked on it, so you can step through the whole event without it leaving the field.

On the chart they look the part: the Moon turns dark brick red as the Earth's shadow crosses it, the daytime sky drops to a deep blue in a total eclipse of the Sun and the planets come out, and Mercury and Venus cross the Sun as black dots.

The Moon half inside the Earth's shadow, brick red on one side and still sunlit on the other, among the stars — The lunar eclipse of 7 September 2025 from Padova, half an hour before totality: the shadow on one side, sunlight still on the other.

06 Images

Your photographs, laid exactly where they belong

Open a plate-solved FITS — or a PNG or TIFF with the file its solver wrote — and Sky42 lays it on the sky in its true place, size and orientation. Drag a file onto the window and it is there. The catalogue stars sit on the stars of the picture, and the chart goes on around it: names, symbols, grids, the tracks of moving bodies.

Each image has its own black point, midtones, white point and opacity, and a button that sets the clock to when it was taken, so that the asteroids and comets in the frame can be named. Where two images overlap, the brighter one shows at each point, so a mosaic stays a mosaic.

Add a folder once, and every solved picture in it and its subfolders appears when you look that way — a few at a time, and never more than the computer can hold. The Images button hides and shows them all, to compare a photograph with the chart.

FITS, PNG, TIFFwith the solution they carry or the one beside them

0.05″a pixel in the Hubble frames shown here

Three overlapping Hubble frames of the spiral galaxy NGC 3621 laid on the Sky42 chart, inside the catalogue ellipse of the galaxy, with catalogue stars drawn over the stars of the photograph — Three Hubble frames of NGC 3621 on the chart, in a 12′ field: the galaxy's catalogue ellipse round them, and Gaia's stars on the stars of the photograph. Data: NASA/ESA Hubble Space Telescope, programme 17189 (PI Thilker), 28 December 2023.

07 Deep-sky photographs

What the galaxy on the chart looks like through a great telescope

Press X and the galaxies, nebulae and globular clusters of the NGC and IC catalogues get a black-and-white photograph, laid exactly in place, from the Digitized Sky Survey: the scanned glass plates of the great Schmidt telescopes at Palomar and Siding Spring. For each object Sky42 picks the red or the blue plate, whichever shows it better.

A photograph appears as you zoom in far enough for it to be worth seeing, and the chart's own stars and names stay on top of it. Under a bright sky it fades the way the faint light of a real galaxy would: dimmer under the Moon, gone in twilight and daylight.

Sky42 comes with the photographs of the Messier objects. All the others are one download from the Catalogue Manager.

10,526galaxies, nebulae and globular clusters photographed

76Messier objects included with Sky42

The galaxies M81 and M82 in Ursa Major on the Sky42 chart, each with its black-and-white photograph laid in place among the chart's stars: the spiral arms of M81 below, the edge-on M82 above, and NGC 3077 in the corner — M81 and M82 in Ursa Major, in a 1.3° field, with NGC 3077 in the corner. Digitized Sky Survey — Provenance: Data taken by ROE and AAO, CalTech, Compression and distribution by Space Telescope Science Institute. Copyright: STScI, ROE, AAO, UK-PPARC, CalTech, National Geographic Society. Full copyright notice.

Also in 1.2

Asteroids in other centuries

The six thousand asteroids that can reach magnitude 15 are drawn where they really were in any year from 1600 to 2500 — and for a single year, a small file adds a hundred and fifty thousand down to magnitude 18. On the day Giordano Bruno died, 17 February 1600, Vesta was a sixth-magnitude point in Leo.

A starting view that looks like the sky

Sky42 opens facing south over a straight horizon, 60° wide, with the ground, the deep sky and the Milky Way on. The first time, it places you in the largest town of your computer's time zone until you choose your own place.

Time that runs both ways

Play, pause, faster and slower, from real time to a year a second — forwards or backwards. The toolbar shows the Julian Date beside the local time, and typing into any of the three moves the other two.

Turn the field, or mirror it

Drag with the right mouse button to turn the chart to any angle, to match a photograph or the eyepiece, and mirror it for a star diagonal. Reset View brings back the standard one when you are lost.

More in the Object panel

When the object rises, culminates and sets, the best time to observe it in a dark sky, its airmass and what the air takes from its brightness, its distance from the Moon, and the constellation it is in.

A search box that takes more

Type a position instead of a name, a Greek letter written out — zeta Her, alf CMa — or a town in your own language when choosing your site: Venezia finds Venice, and 北京 finds Beijing.