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Wednesday, 28 June 2017 00:12

As Hipparchus did more than two thousands years ago...

Roland Boninsegna

"EAAE Summerschools" Working Group

Centre Scientifique Fleurus Sivry (Belgium)


During an observing session, it is better to propose, to the students who are waiting their turn at the telescope, an active observation with the naked eye. A first approach without instrument, consist in ranking some bright stars of a constellation according to their visual brightness, as Hipparchus did more than two thousands years ago. It is possible, by this way, to stimulate active astronomical observation, to compare stars of different colours with the sensibility of human eye and to notice the effect of the colour absorption by the atmosphere.


Stimulate an active observation

Become aware of the different sensibility of the human eyes

Become aware of the extinction due to atmospheric absorption


The main northern constellations are known for more than five thousands years. However, the first sky catalogue was created by Eudoxus (408 - 359 BC). Inside was included 43 constellations and 47 bright stars. Hipparchus (128 BC), observed the position and the brightness of 850 stars, this catalogue will be extended and updated by Claudius Ptolemy (150 AD). This work will be the reference all along the Middle Age. In these old catalogues, the stars brightness was roughly represented by a number from 1, for the most brilliant, to 6 for the faintest stars visible to the naked eye. From then, the catalogues were becoming more and more important and precise with the help of different tools and techniques.


During a night observing session, it is of some utility to propose the students, waiting to look through the telescope, an simple observing activity. One of these is the same which occupied Hipparchus and Claudius Ptolemy in ancient time: the brightness ranking of stars within a constellation. A simple drawing of a constellation with the brightest stars is presented to the observer (see fig. 1). The stars to rank are labelled using letters. A scale of the field of view serve as comparison with natural measures (a fist outstretched represents 10° in the sky).

The first task is to locate the constellation in the sky, alone or with someone's help. Then, spot each star labelled. Rank them, in decreasing order of brightness. If some stars seems to be of equal brightness, you can record that equality (B=D) on the same line of the table. Try to complete the classification in 15 minutes maximum. Write the time (local) at the end of your classification. Complete the observing sheet in adding your name. A second table is provided in order to write a clear classification if necessary.

In case of bad weather, it is possible to propose an indoor exercise using a projected slide of the constellation.

Correction and discussions

After the observation, the corrected tables are presented (see fig. 2). The stars presented on the same line can be considered as of equal brightness for the human eye. The name, V magnitude and spectral type is presented for each star. The photoelectric V magnitude is slightly different from the v magnitude of the human eye, but can be compare crudely. The spectral type can be compared with the visual colour. The colour of a star (related to his superficial temperature) can be seen as pale blue, for the hottest, to orange or red for the coldest. A simple way to link spectral types and visual colours is as follows:

O-B-A: blue; A-F: white; F-G: yellow; G-K: orange; M-R-N-S: red

Stars of almost the same brightness can be classified in a different way by the observers due to the sensibility difference for each human eye. If the stars of a constellation were observed at a low altitude above horizon (less than 20°), the effect of atmospheric absorption could be detect. For example, a blue star of almost the same V brightness than a yellow or red one, will be perceived fainter for the same low altitude (See for example the stars E and G of Auriga).

The magnitude scale

The Greek astronomer Hipparchus classified the brightness of the stars he could see, in six "degrees". In the middle of the 19th century, an accurate scale was defined in respect with the basements of Hipparchus and Claudius Ptolemy. The ratio of brightness between a star of magnitude 1 and one of magnitude 6 is equal to 100: so, one magnitude difference correspond to a ratio of intensity of 2.512 and a difference of n magnitude can be expressed (2.512)n . An increasing in magnitude coincide with a decreasing in brightness. That scale was extended from the faintest detected objects, to the brightest (see fig. 3). Hence, the faintest stars recorded by the Hubble space telescope are around magnitude 30. Concerning the brightest objects, it was necessary to enlarge the scale towards magnitude 0 and beyond towards negative numbers: the brightest star in the night sky is Sirius (a Canis Major) reaching magnitude -1.4, our star, the Sun, hold the leadership with -26.7! Nowadays, it is possible to measure brightness differences as small as 0.001 magnitude.

  1. Locate the constellation with a sky chart. Ask for help if necessary.
  2. Comparing with the chart below, spot each star labelled on the sky.
  3. Rank the constellation stars by decreasing order of brightness (the most luminous, first …). If you estimate that two (or more) stars have the same brightness, you can write down an equality (B=D) on the same line. Don't forget to write down the time when you begin the classification.
  4. Write also the time when the classification is over.
  5. Recopy the ranking. Write your name and first name overleaf.

The Charioteer - Auriga - Aurigae - Aur

Figure 1 - Example of chart proposed for the ranking of the brightest stars of a constellation.
Figure 1 - Example of chart proposed for the ranking of the brightest stars of a constellation.
* Name Magnitude Color   Classification Magnitude
A α 0.08 G8   A 0.08
B β 2.99 F0   F 1.65
C η 3.17 B3   H 1.90
D ζ 3.75 K4   G-E 2.62-2.69
E ι 2.69 K3   B 2.99
F β Tau 1.65 B7   C 3.17
G θ 2.62 A0   D 3.75
H β 1.90 A2      

Figure 2 - Classification of the brightest stars of the constellation Auriga.

If you wish charts for more constellations, you can download them from this webpage: http://users.skynet.be/boninsegna