
41-6 Time and Frequency
Before the invention of atomic clocks and the acceptance of atomic time, the second was dened by
usi
ng
as
tronomical
ti
me
sc
ales
. e as
tronomical
de
nitions
of th
e
se
cond
we
re
ve
ry
di
erent
fr
om
th
e
ato
mic
de
nition
. In
stead
of mu
ltiplying
th
e
pe
riod
of a sh
ort
ato
mic
ev
ent
to fo
rm
a lo
nger
ti
me
in
ter-
val,
th
e
ea
rly
de
nitions
of th
e
se
cond
we
re
ba
sed
on di
viding
th
e
pe
riod
of a lo
ng
as
tronomical
ev
ent
to
fo
rm
a sh
orter
ti
me
in
terval
. e
re
wa
s
no o
cial
me
trological
de
nition
of th
e
se
cond
un
til
th
e
SI wa
s
fo
rmed
in 19
60
. Ho
wever,
pr
ior
to th
at
da
te,
th
e
se
cond
wa
s
ob
tained
by di
viding
th
e
me
an solar day
or
th
e
av
era