Download Solution Sheet Lab 1

ASTRONOMY 205.1 GENERAL ASTRONOMY I
“Solutions” Sheet
Student Name
Laboratory Exercise 1. The Length of a Sidereal Day
Date Experiment Performed
Purpose. To determine the length of the sidereal day (the “star” day) from an image of the circumpolar
region of the sky. The length of the sidereal day is defined as the time interval between two successive
transits of the vernal equinox across the meridian. It is time based upon the Earth’s rotation on its axis
with respect to the celestial sphere, or stars, rather than with respect to the Sun, as is the case for solar
time. In order to measure the length of the sidereal day, it is necessary to measure the apparent motion of
the stars around the sky. Since that is difficult to do for a full day, it is convenient to image the motion of
the stars for a shorter, but well-established, time duration.
Procedure. See Laboratory Manual.
Equipment.



image of circumpolar star trails (from Laboratory Manual, see next page)
protractor
a pocket calculator
Data.
Angles obtained from careful measurements made with a protractor, with setting precision estimated to be
±0°.1 to ±0°.2.
Angle Measurements for Six Star Trails
15m Exposure
A
Star 1
Star 2
Star 3
Star 4
Star 5
Star 6
20m Exposure
B
110m Exposure
C
95m Exposure
C–A
90m Exposure
C–B
Image of Circumpolar Star Trails
Estimates for Length of Sidereal Day
360 
 110 m
C
360 
 95 m
CA
360 
 90 m
CB
Star 1
Star 2
Star 3
Star 4
Star 5
Star 6
Average of “Best” Values

Largest – Smallest Value
Estimated Length of Sidereal Day =
1
2
±
=
minutes
Accepted Length of Sidereal Day = 1436 minutes.
Accepted Length of Solar Day = 1440 minutes.
Earth’s Equatorial Velocity =
2R
Sidereal Day

=
2 (6378 km)
Sidereal Day (seconds)

km
s
km s–1.
Conclusions.
1. The estimated length of the sidereal day
accepted value for the length of the sidereal day, to within experimental uncertainty.
with the
2. The estimated length of the sidereal day
accepted value for the length of the solar day day, to within experimental uncertainty.
with the
3. The estimated equatorial velocity for the Earth is
km s–1.
Discussion.
(Do you expect the lengths of the solar day to agree with the value obtained for the length of the sidereal
day? Why or why not?)