230
4 Human Perception of the Pleiades
Although most people see six stars, some see far more. For example, the first nonAboriginal Australian astronomer, William Dawes, claimed to be able to see 13
stars (Burnham Jr., 1978), so evidently was able to see stars fainter than sixth magnitude. There are several other accounts of individuals with exceptional eyesight
who can see large numbers of stars. Nevertheless, most people see six stars, a few
can see eight, and rarely, those with exceptional eyesight see even larger numbers of
stars. However, there is significant disagreement over which stars are included in the
Seven Sisters.
The “Seven Sisters” of Greek mythology are unambiguous and are indicated by
asterisks in Table 1. Most modern people with good eyesight can easily see the
brightest five: Alcyone, Merope, Electra, Maia, and Taygeta, all of which are
mv = 4.3 or brighter. Atlas (mv = 3.6) is often included as one of the Seven Sisters
even though, in Greek mythology, Atlas is the father of the sisters. Pleione is the
next brightest star (at mv = 5.05), and so is the obvious candidate for the seventh
star, although in Greek mythology she is the mother of the Seven Sisters. However,
Pleione is very close to the bright star Atlas, making it hard to see, as will be discussed below. The remaining three stars (Celaeno, 18 Tau, and Asterope are all very
faint (at mv = 5.45 or fainter), close to the human limit of sensitivity, and cannot be
seen by most people. Thus the six stars of the Seven Sisters as pointed out by many
contemporary observers (e.g. King, 2014) are Alcyone, Merope, Electra, Maia,
Taygeta, and Atlas, with a seventh, Pleione, just visible to those with exceptional
eyesight.
5 The Physiology of Seeing Stars
There are several distinct physiological effects that limit the human perception
of stars.
First, the sensitivity of the human eye limits the vision of most people, in a dark
sky, to stars brighter than sixth magnitude. Indeed, the system of measuring a stars
brightness by its “magnitude” was initially based on defining a sixth magnitude star
as one that was just visible to the human eye (Heifetz & Tirion, 2004).
Second, the resolving power of the eye, in bright light, is limited to about
1 arcmin (which is the distance between the arms of the E on the bottom line of an
optometrist’s Snellen chart) (Yanoff & Duker, 2009), so that two stars closer than
this will appear as one. This is a few times worse than might be expected according
to the Rayleigh criterion for a diffraction-limited aperture, because of aberrations in
the eye.
If these were the only two effects, then most humans would see the ten stars
listed in Table 1, as they are all brighter than sixth magnitude and are all separated
R. P. Norris and B. R. M. Norris
4 Human Perception of the Pleiades
Although most people see six stars, some see far more. For example, the first nonAboriginal Australian astronomer, William Dawes, claimed to be able to see 13
stars (Burnham Jr., 1978), so evidently was able to see stars fainter than sixth magnitude. There are several other accounts of individuals with exceptional eyesight
who can see large numbers of stars. Nevertheless, most people see six stars, a few
can see eight, and rarely, those with exceptional eyesight see even larger numbers of
stars. However, there is significant disagreement over which stars are included in the
Seven Sisters.
The “Seven Sisters” of Greek mythology are unambiguous and are indicated by
asterisks in Table 1. Most modern people with good eyesight can easily see the
brightest five: Alcyone, Merope, Electra, Maia, and Taygeta, all of which are
mv = 4.3 or brighter. Atlas (mv = 3.6) is often included as one of the Seven Sisters
even though, in Greek mythology, Atlas is the father of the sisters. Pleione is the
next brightest star (at mv = 5.05), and so is the obvious candidate for the seventh
star, although in Greek mythology she is the mother of the Seven Sisters. However,
Pleione is very close to the bright star Atlas, making it hard to see, as will be discussed below. The remaining three stars (Celaeno, 18 Tau, and Asterope are all very
faint (at mv = 5.45 or fainter), close to the human limit of sensitivity, and cannot be
seen by most people. Thus the six stars of the Seven Sisters as pointed out by many
contemporary observers (e.g. King, 2014) are Alcyone, Merope, Electra, Maia,
Taygeta, and Atlas, with a seventh, Pleione, just visible to those with exceptional
eyesight.
5 The Physiology of Seeing Stars
There are several distinct physiological effects that limit the human perception
of stars.
First, the sensitivity of the human eye limits the vision of most people, in a dark
sky, to stars brighter than sixth magnitude. Indeed, the system of measuring a stars
brightness by its “magnitude” was initially based on defining a sixth magnitude star
as one that was just visible to the human eye (Heifetz & Tirion, 2004).
Second, the resolving power of the eye, in bright light, is limited to about
1 arcmin (which is the distance between the arms of the E on the bottom line of an
optometrist’s Snellen chart) (Yanoff & Duker, 2009), so that two stars closer than
this will appear as one. This is a few times worse than might be expected according
to the Rayleigh criterion for a diffraction-limited aperture, because of aberrations in
the eye.
If these were the only two effects, then most humans would see the ten stars
listed in Table 1, as they are all brighter than sixth magnitude and are all separated
R. P. Norris and B. R. M. Norris