How to learn the sky

On a clear night with no Moon, far from any town, a person with good eyesight can see about 2500 stars above the horizon at once. Over the whole sky there are about 5000 stars bright enough to see with the naked eye, and nobody who knows the sky well has learned them one at a time. An experienced observer who steps outside recognises the sky the way we recognise a face, from a few strong features. On a January evening in the northern hemisphere these might be the three stars of Orion's belt in a short straight row, the orange star Betelgeuse above them and the brilliant Sirius below them and to the left. Once those are found, the rest of the sky is found from them. In this section we see what the observation round expects you to know about the sky, how the sky is divided into constellations and how its stars are named, and a way of learning it that starts from a handful of bright patterns and works outward.

O1.1.1What the observation round asks

The observation round carries a quarter of the total marks (Section S1), and its form changes with each host country and with the weather. There may be questions under the real night sky, in a planetarium where the sky is projected on a dome, at a telescope, or on printed sky maps. Nearly all of them start from the same skill, which is knowing where you are in the sky. A question may ask for the name of the constellation that a pointer shows on the dome, for a given star to be found with a telescope, or for the stars on a map that a planetarium sky shows above the horizon.

The practical part of the IOAA syllabus lists observation with the naked eye, the use of sky maps and catalogues, and the estimation of coordinates, of brightness and of angles between stars. It adds that named stars in questions come from the list of names approved by the International Astronomical Union (IAU), and that learning the whole list by heart is not expected. That list holds several hundred names. This chapter teaches the brightest stars and the stars that outline the constellations, and for each one a way of finding it from stars you already know.

O1.1.2Constellations, asterisms and Bayer letters

People have grouped the brighter stars into figures since ancient times. In 1922 the IAU fixed the list of these constellations at 88, and in 1930 it published boundaries for them, drawn along lines of right ascension and declination as they were in the year 1875. A constellation today is the whole area of sky inside its boundary, so every star or galaxy lies in exactly one constellation, and the Moon and the planets move from one to the next. Precession (Section 2.7) has turned the grid of right ascension and declination a little since 1875, so the boundaries no longer run exactly along today's lines.

The lines that join stars into a figure have no official standing. Atlases, planetarium programs and the charts in this course draw some figures differently, and the exam may use any of them. The figure is only an aid for remembering the stars, and the stars are what we learn.

A pattern of stars that is not one of the 88 constellations is called an asterism, and some asterisms are better known than the constellations they belong to. The Big Dipper, called the Plough in Britain, is 7 bright stars that make the back and tail of Ursa Major, the Great Bear. The Summer Triangle joins three bright stars in three different constellations, Vega in Lyra, Deneb in Cygnus and Altair in Aquila. The Great Square of Pegasus has three of its corners in Pegasus and the fourth, Alpheratz, in Andromeda.

Many bright stars have a proper name, such as Betelgeuse or Sirius, which usually comes from Arabic, Greek or Latin. Almost all of them also have a Bayer designation, a Greek letter followed by the Latin name of the constellation in the genitive case. Betelgeuse, for example, is α Orionis, which we write α Ori for short, using the three-letter abbreviation of the constellation that the IAU fixed along with the names. The letters come from a star atlas that Johann Bayer published in 1603, and every olympiad map and table uses them, so the Greek alphabet in the table below has to be learned by heart.

Bayer gave the letters roughly in order of brightness. The brightness of a star is measured by its magnitude, on a scale that runs backwards, so that a brighter star has a smaller magnitude. Bayer sorted the stars of each constellation into a few classes of brightness, and within a class he often went along the figure, so his order is only rough. In Gemini, Castor is α and Pollux β, but Pollux is the brighter of the two, at magnitude 1.16 against 1.58 for Castor. In the Big Dipper the letters simply run along the figure, starting with α and β at the front of the bowl, going round the bowl to δ where the handle joins it, and ending with η at the tip of the handle.

O1.1.3Anchors and star-hops

We start with a few anchors, bright patterns that are hard to mistake for anything else. Each is high in the evening sky in a different season, and the table below gives the date on which it crosses the meridian, and so is highest in the sky, at 21:00 local mean time. A star crosses the meridian about 2 hours earlier for each month after that date and 2 hours later for each month before it (Section 2.1), so each anchor is well placed in the evening for two or three months around its date. Clock time can differ from local mean time by an hour or more (Section 2.3).

From an anchor we find other stars by star-hopping. We follow a line through two stars we know and go on along it by a distance that we measure with the gap between the two stars or with a hand held out at arm's length, as Section O1.1.4 describes. The best-known hop is the one from the Big Dipper to Polaris, the Pole Star, which lies within a degree of the north celestial pole. The two stars at the front of the bowl, Merak and Dubhe, are called the Pointers, because the line from Merak through Dubhe, continued about five times their separation, reaches Polaris. On the other side of Polaris, about as far from it as the Big Dipper, is the W of Cassiopeia, and a line from Megrez, where the handle joins the bowl, through Polaris leads to it.

In the southern hemisphere, where Polaris is below the horizon, the anchor of the polar sky is the Southern Cross, Crux, with the two bright stars Alpha and Beta Centauri beside it. No bright star marks the south celestial pole, and Section O1.6 shows how the Cross and the two stars lead to it.

O1.1.4Measuring angles and brightness by eye

A hand held out at arm's length is a ruler for the sky that every observer carries. A finger, a fist or a spread hand covers an angle that follows from the small-angle approximation of Section S3.

The table below lists pairs of bright stars for measuring a hand against. The full Moon, 0.52° across, is the size of a pea held at arm's length, much smaller than most people expect.

Brightness is judged in the same way, against stars we know. On the magnitude scale a difference of 5 magnitudes is a factor of exactly 100 in brightness, so one magnitude is a factor of ${100}^{1/5}=2.512$, as Chapter 4 explains. A few stars make good reference points. Vega has magnitude 0.03, very nearly 0, and Polaris 1.97. The three stars of Orion's belt and six of the seven stars of the Big Dipper are within half a magnitude of 2, and the seventh, Megrez, is about a magnitude fainter. Sirius, the brightest star of the night sky, has magnitude −1.44, and the faintest stars the naked eye sees under a dark sky are near magnitude 6. We estimate the magnitude of a star by finding two stars of known magnitude near it, at about the same height above the horizon, one a little brighter and one a little fainter, and placing it between them. Chapter O2 gives more practice in this.

O1.1.5How to practise

Learning the sky is memory work, and memory work goes best in short daily sessions spread over weeks. The route of this course sets aside about 20 minutes a day for the sky from the first day, alongside the theory. This chapter takes the sky one region at a time, starting with Orion in Section O1.2, and each region has drills in which you find its stars on a chart with no names on it.

Every answer in a drill goes into the daily round on the Sky page. A star you find comes back after 1 day, then after 3, 7, 14 and 30 days as long as you keep finding it, and a star you miss is due again the same day. The sky trainer asks for the same stars in other ways, by name and by Bayer letter, on charts that can be turned to any angle as the sky on a planetarium dome can be.

The charts in the course come in two styles. One is the dark dome of a planetarium. The other is a printed sky map like those of the sky-map round, black on white, with north at the top and a grid of right ascension and declination. The same constellation looks different on the two, so we practise on both. When the weather allows, the same stars should be found outside as well, since the real sky has no lines on it and far more faint stars than a chart. Drawing a constellation from memory and then checking it against a chart is another quick test.