Angular Measurement

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Transcript Angular Measurement

Background from Chapter 1:
•Scientific Method
How Do We Know All This?
Scientific Method
Allows theories to evolve through a
combination of observations, theoretical
reasoning, and prediction—which in turn
suggests new observations.
How Do We Know All This?
 experimental verification
 continual testing allows theories to be
 Occam’s razor
 why make a new theory when you can
amend an old one?
Chain of Scientific Method
Scientific Method
Gravity is a theory?
Yes, however, this theory has guided
scientists’ calculations with technology from
the days of Sir Isaac Newton to Space
Travel calculations
The natural thing to do with observations, for
humans, is to give them an order.
Long ago, people have recognized patterns
in the sky and applied mythology . . .
A human grouping of stars in the night
sky into a recognizable pattern
Constellations Near Orion The region of the sky near Orion, together with some
neighboring constellations.
Some prominent stars are labeled in lowercase letters. The 88 constellations span
the entire sky, so that every astronomical object lies in precisely one of them
Benefits to mapping the night sky?
Benefits to mapping the night sky?
Astrology (i.e. Miss Cleo)
Now remember that space is 3-D
1,000 light-years
celestial sphere
Surrounding earth we
see a “canopy of stars
resembling an
astronomical painting on
a heavenly ceiling.”
light year
The distance that light, moving at a constant
speed of 300,000 km/s, travels in one year.
One light year is about 10 trillion kilometers
Angular Measurement -- 1
•A full circle contains 360 degrees (360°).
•Thus, the half-circle that stretches from
horizon to horizon, passing directly
overhead and spanning the portion of the
sky visible to one person at any one time,
contains 180°.
Angular Measurement -- 2
•Each 1° increment can be further
subdivided into fractions of a degree,
called arc minutes.
•There are 60 arc minutes (written 60´) in
one degree.
•Both the Sun and the Moon project an
angular size of 30 arc minutes on the sky.
Your little finger, held at arm’s length, does
about the same, covering about a 40' slice
of the 180° horizon-to-horizon arc.
Angular Measurement -- 3
•An arc minute can be divided into 60 arc
seconds (60´´).
•Put another way, an arc minute is 1/60 of a
degree, and an arc second is 1/60 x 1/60 = 1/3600
of a degree.
• An arc second is an extremely small unit of
angular measure—it is the angular size of a
centimeter-sized object (a dime, say) at a
distance of about two kilometers
(a little over a mile).
Angular Measurement
Angular Measurement -- 4
The angular size of an astronomical
object depends on:
1.its actual size
2. its distance from us
Angular Measurement -- 5
• For example, the Moon, at its present distance from Earth,
has an angular diameter of 0.5°, or 30´.
• If the Moon were twice as far away, it would appear half as
big—15´ across—even though its actual size would be the
• Thus, angular size by itself is not enough to determine the
actual diameter of an object—the distance must also be
Solar Days
•24-hour solar day, is our basic
social time unit. The daily
progress of the Sun and the
other stars across the sky is
known as diurnal motion.
•As we have just seen, it is a
consequence of Earth’s rotation.
•But the stars’ positions in the
sky do not repeat themselves
exactly from one night to the
•Each night, the whole celestial
sphere appears to be shifted a
little relative to the horizon,
compared with the night before
Sidereal Days
•Because of this shift, a day measured by the
stars—called a sidereal day after the Latin
word sidus, meaning "star"—differs in length
from a solar day.
•Evidently, there is more to the apparent
motion of the heavens than simple rotation.
Sidereal Days
•sidereal day
The time needed for a star on
the celestial sphere to make one
complete rotation in the sky.
Night Sky
•Typical Night Sky (a) A typical summer sky above the United States.
Some prominent stars (labeled in lowercase letters) and constellations
(labeled in all capital letters) are shown. (b) A typical winter sky above
the United States.
Night Sky
•The 12 constellations through which the Sun passes
as it moves along the ecliptic—that is, the
constellations we would see looking in the direction of
the Sun, if they weren’t overwhelmed by the Sun’s
light—had special significance for astrologers of old.
These constellations are collectively known as the
Night Sky
•The Zodiac The view of the night sky changes as Earth moves in its
orbit about the Sun. As drawn here, the night side of Earth faces a
different set of constellations at different times of the year. The twelve
constellations named here comprise the astrological zodiac.