Even though time has existed since the beginning

of, well, time, it was still necessary to invent it.

BY RAYMOND SHUBINSKI

TIME

There are many ways to

ask, “What is the time?”

Astronomers can use solar

standard time, mean solar

time, sidereal time, Universal

Time, or Julian Date and its

many modified forms.

Astronomers describe three

different types of twilight, the

equation of time, 24 time

zones, and an astronomical

day. Understanding these dif-

ferent “times” gives us a better

idea of our relationship with

the sky above, and the spin-

ning Earth on which we live.

Early civilizations developed

two types of calendars. The

oldest is lunar in nature. It

might seem more logical for

the Sun to have been the first

timekeeper, but archaeologists

have found bones of mam-

moths and other animals

dating over 20,000 years old

that appear to have carvings

recording phases of the Moon.

During that period of human

history, hunters tracking

game needed to know how

long they had been gone from

their camp, making the Moon

the obvious choice to track

the passage of time.

It would be millennia

before the Sun replaced the

Moon in our modern calen-

dar. This is because Earth and

the Moon are involved in a

cosmic mashup that is diffi-

cult to untangle. Most ancient

cultures heralded the begin-

ning of the month when the

thin crescent or “New Moon”

could be seen after sunset.

There are 354 and a fraction

days in a lunar year with 12

lunar months. Earth, how-

ever, revolves around the Sun

every 365.242 days. While

this was not a problem in a

purely ceremonial or religious

calendar, trying to mesh these

two calendars was impossible.

The solution to this issue

was proposed by Sosigenes of

Alexandria, Cleopatra’s court

astronomer and arguably the

most inf luential astronomer

in all of history. Julius Caesar

Defining

TIME AND ASTRONOMY ARE INSEPARABLE. Humans have

been using the motions of the stars, Sun, and Moon for thousands of years

to regulate their hunting, crops, religion, and lives in every way. And as

astronomy developed, so did the need for more precise timekeeping.

54 ASTRONOMY • DECEMBER 2021