4.3. UNITS

25

Table 4.2 Additional allowed units.

Quantity

Unit

Meaning

Notes

plane angle

degree of arc

π/180 rad

deg

minute of arc

1/60 deg

arcmin

second of arc

1/3600 deg

arcsec

milli-second of arc

1/3 600 000 deg

mas

time

minute

60 s

min

hour

60 min = 3600 s

h

day

86 400 s

d

†

year (Julian)

31 557 600 s (365.25 d), peta a (Pa) forbidden

a

†

year (Julian)

a is IAU-style

yr

energy∗

1.6021765 × 10-19 J

†

electron volt

eV

10-7 J

‡

erg

erg

(2     2 )^2

πe

1

mec2 = 13.605692 eV

rydberg

Ry

(hc)^2

2

mass∗

1.9891 × 1030 kg

solar mass

solMass

1.6605387 × 10-27 kg

unified atomic mass unit

u

3.8268 × 1026 W

luminosity

Solar luminosity

solLum

10-10 m

length

‡

angstrom

Angstrom

6.9599 × 108 m

Solar radius

solRad

1.49598 × 1011 m

astronomical unit

AU

9.460730 × 1015 m

light year

lyr

3.0857 × 1016 m

†

parsec

pc

events

count

count

count

ct

photon

photon

photon

ph

10-26 W m-2 Hz-1

flux density

†

jansky

Jy

†

(stellar) magnitude

mag

1010/(4π) photons m-2 s-1 sr-1

†

rayleigh

R

10-4 T

magnetic field †‡

gauss

G

area

(image/detector) pixel

pixel

(image/detector) pixel

pix

10-28 m2

†‡

barn

barn

Miscellaneous units

1

× 10-29 C.m

debye

D

3

relative to Sun

e.g., abundances

Sun

(detector) channel

chan

numerous applications

(including the 1-d analogue of pixel)

bin

3-d analogue of pixel

voxel

† bit

binary information unit

† byte

(computer) byte

8 bit

Analog-to-digital converter

adu

beam area of observation

as in Jy/beam

beam

† - addition of prefixes for decimal multiples and submultiples are allowed.

‡ - deprecated in IAU Style Manual [19] but still in use.

∗ - conversion factors from CODATA Internationally recommended values of the fundamental

physical constants 2002 (http://physics.nist.gov/cuu/Constants/).

FITS Standard