Physics 162
Elementary Astronomy
Dr. Fortner
FW 204
MWF
mfortner@niu.edu
Web Pages
• My Page: www.niu.edu/~mfortner/
• PHYS 162 syllabus and lectures:
nicadd.niu.edu/~fortner/course/phys162/
• Webassign: www.webassign.net
• Observatory: www.niu.edu/physics/observatory/
Course Topics
• The course is divided into four broad topics.
– Foundations - Tools and rules
– Planets - Earth and its neighbors
– Stars - How the sun and other stars work
– Universe - Galaxies and beyond
• Each topic includes a written assignment and a test.
• Each week includes an online homework assignment.
Why Astronomy?
• Astronomy is as old as humans have looked up at the sky.
• Astronomy is the science of stars and other objects in
space.
– Physics, chemistry, geology, even some biology
– Laboratory for conditions beyond earth-based
experiments
• Astronomy remains newsworthy today.
– News stories from NASA and other countries
– Movies and popular culture
Measurement and Units
• Scientific measurement requires standard units.
– English units – foot, pound, second
– Metric units – meter (m), kilogram (kg), second (s)
– Derived units – cubic meter (m3), watt (W)
– Alternate units – hour (h), light year (ly)
• The metric system has two major strengths.
– Widespread use in science
– Prefixes based on powers of ten
Scientific Notation
• In science some measurements can be very large or very
small compared to the base unit.
• Scientific notation uses powers of 10 to represent decimal
places.
– Positive for large values: 456000 = 4.56 x 105
– Negative for small values: 0.00753 = 7.53 x 10-3 (only
one non-zero value to the left of the decimal place)
SI Prefixes
• Prefixes on units are used to
represent powers of ten.
• Prefixes denote powers of ten
from -18 to +18 mostly in steps
of three.
• For example a kilometer (km) is
103 meters or 1000 meters.
•
•
•
•
•
•
•
Most Common
nano (n) 10-9
micro () 10-6
milli (m) 10-3
centi (c) 10-2
kilo (k) 103
mega (M) 106
giga(G) 109
• Common, but the power is not a
factor of three.
Unit Conversion
• Conversion between units must be of
the same type
– Length conversion: 1 in = 2.54 cm
– Time conversion: 1 h = 3.6 x 103 s
• No conversion between different
types of units.
– 1 m3 not equivalent to any
number of m
Significant Figures
• Any value is expressed in some number of digits.
• The number of digits (without left side zeroes) is the
number of significant figures.
• With no decimal point, skip right side zeroes.
–
–
–
–
–
38
5.06
0.0041
7,000.
2,000
2 digits, 2 significant figures
3 digits, 3 significant figures
5 digits, 2 significant figures
4 digits, 4 significant figures
4 digits, 1 significant figure
One Ton
Assume the density of a rock is
three times that of water. How
many centimeters across is a one
metric ton (1000 kg) rock?
• The rock has a density of 3 x
103 kg/m3.
• The volume is
– 103 kg/(3 x 103 kg/m3)=0.3 m3
• Estimate that the rock is a
sphere, V = (4/3)  r3
– d = 2r = 2 (3V/4 )1/3
– d = 0.9 m x 100 cm/m = 90 cm