Naked Eye Limits

In the case of the human eye, the closest pair of stars which can be seen unaided depends on the diameter of the pupil. This, in turn, depends on the lighting conditions and when fully dark-adapted the pupil may be 6 or 7 mm in diameter, suggesting that the limit of resolution from the Airy formula (see Chapter 10) is about 20'' but the presence of aberrations in the eye and the low light levels from the night sky conspire to reduce the effective resolution to about 2.5'.

In practice a normal pair of eyes should be able to see the stars 81 and 82 Tauri at 5.5' without difficulty and some may be able to make out e1 and e2 Lyrae at 3.6'. The ability to resolve naked-eye pairs tends to deteriorate with age, and younger eyes will probably do better, although practice undoubtedly enhances keenness of vision. Sight, like hearing, or any of the five senses, can be improved with experience. Table 1.1 contains a short list of bright wide pairs which, it is suggested, can be used as a test of naked-eye resolving power. In some of the cases, both stars have a Bayer letter or Flamsteed number and these are used as the main identifier. The positions are given for equinox 2000.0 together with the date of the most recent measure, the visual magnitudes of both stars and the position angle and separation of the pair. Most of these pairs are the results of chance alignment.

Table 1.1

. Some naked-eye double stars.

RA 2000

Dec 2000

Pair

Epoch

PA (°)

Separation ("

Va

Vb

0318.2

-6230

z Ret

1991

217

309.3

5.24

5.33

0425.4

+2218

K Tau

1991

173

340.2

4.21

5.27

0428.7

+1552

0 Tau

1991

347

337.2

3.40

3.84

0439.3

+1555

a Tau

1991

194

437.5

4.67

5.08

0718.3

-3644

Jc 10 Pup

1991

98

240.0

4.65

5.11

1208.4

-5043

8 Cen

1991

325

269.1

2.58

4.46

1450.9

-1603

a Lib

1991

314

230.7

2.75

5.15

1622.4

+3348

v CrB

1984

165

359.5

5.20

5.39

1844.3

+3940

e Lyr

1998

174

210.5

4.59

4.67

1928.7

+2440

6-8 Vul

1991

28

422.9

4.44

5.82

2013.6

+4644

31-32 Cyg

1998

324

330.7

3.80

4.80

2018.1

-1233

a Cap

1984

290

381.4

3.80

4.20

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