Chapter 4
Time
Abstract The measurement of time was one of the first applications of astronomy,
and many of observational astronomy’s underpinnings are related to time. Knowing
the Julian date and local sidereal time is key in identifying what objects can be
observed and when.
4.1 Introduction
Our modern lives are ruled by time; either the “tick tick tick” of the clock or, more
commonly now, the fast harmonic hum of a quartz crystal. We work to timetables and
deadlines, and the term “working day” often involves little actual daylight. However,
for the greater part of human existence, when cultures were predominantly hunter–
gatherer, pastoral nomad, and agrarian, time was merely the changing of the seasons
and the rising and setting of the Sun. When to eat, when to sleep, when to move the
herds, when to plant, and when to harvest. Our ancestors noticed that the point of
sunrise on the horizon changed through the year. When the day was at its shortest,
the Sun rose at its furthest point north; when at its longest, the Sun rose at its furthest
point south; and twice a year, when the days were equal in length, the Sun rose at
the midpoint between the northern and southern extremes. The first two events are
known respectively as the winter and summer solstices. The term comes from the
Latin for Sun (sol) and the verb to stand still (sistere). The latter pair are the spring
and autumnal equinoxes, from the Latin aequalis (equal) and nox (night). Great
significance was placed on these events, and great structures, such as Stonehenge in
Great Britain, were erected to observe them. These structures became, in effect, the
first calendars.
Later, with the development of cities, military forces, organised religion, and elite
ruling classes, marking the passage of time and its measurement became increasingly
important to intrasocietal coordination. Within an extended community, the populace
needed to know when to pray, when to change the guard, and when the tax collector
was coming. Time was measured using sundials, water clocks, sand timers, and
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_4
45
Time
Abstract The measurement of time was one of the first applications of astronomy,
and many of observational astronomy’s underpinnings are related to time. Knowing
the Julian date and local sidereal time is key in identifying what objects can be
observed and when.
4.1 Introduction
Our modern lives are ruled by time; either the “tick tick tick” of the clock or, more
commonly now, the fast harmonic hum of a quartz crystal. We work to timetables and
deadlines, and the term “working day” often involves little actual daylight. However,
for the greater part of human existence, when cultures were predominantly hunter–
gatherer, pastoral nomad, and agrarian, time was merely the changing of the seasons
and the rising and setting of the Sun. When to eat, when to sleep, when to move the
herds, when to plant, and when to harvest. Our ancestors noticed that the point of
sunrise on the horizon changed through the year. When the day was at its shortest,
the Sun rose at its furthest point north; when at its longest, the Sun rose at its furthest
point south; and twice a year, when the days were equal in length, the Sun rose at
the midpoint between the northern and southern extremes. The first two events are
known respectively as the winter and summer solstices. The term comes from the
Latin for Sun (sol) and the verb to stand still (sistere). The latter pair are the spring
and autumnal equinoxes, from the Latin aequalis (equal) and nox (night). Great
significance was placed on these events, and great structures, such as Stonehenge in
Great Britain, were erected to observe them. These structures became, in effect, the
first calendars.
Later, with the development of cities, military forces, organised religion, and elite
ruling classes, marking the passage of time and its measurement became increasingly
important to intrasocietal coordination. Within an extended community, the populace
needed to know when to pray, when to change the guard, and when the tax collector
was coming. Time was measured using sundials, water clocks, sand timers, and
© Springer Nature Switzerland AG 2020
M. Gallaway, An Introduction to Observational Astrophysics,
Undergraduate Lecture Notes in Physics,
https://doi.org/10.1007/978-3-030-43551-6_4
45