Astronomical
Distances
Stars that seem to be close may
actually be very far away from each
other.
• The universe is immense in size
• Astronomers use units and
methods of measuring that we are
not used to on Earth
• Parallax is used to measure the
distance to nearby stars
• Parallax: when objects appear to
move or shift due to earth’s orbit
★This method uses trigonometry
★nearby stars = 300 light years or
less
Parallax
Jan.
draw
this
June
•Inverse square law of light is used
for distant stars
•the intensity of light falls off as the
square of the distance from the
source
• Light year is a unit of distance,
NOT time
• Light Year: the distance light
travels in one year (9.5 trillion
km!!!)
★Kilometers are too small to use in
astronomy
★one light-year is equal to
9,500,000,000,000 kilometers.
Light Year
• Parsec is used to measure greater
distances between stars and
galaxies
• 1 parsec = 3.26 light years
• Astronomical Units (AUs) measure
distances between planets (our
solar system)
• AU is the average distance
between the Earth and Sun (1 AU)
★1 AU is about 150 million km
★Mercury is the closest at 0.39 AUs while
Neptune is the farthest at 30.06 AUs
draw
this
1 AU
Sun
E
60 second summary
Review#1 What is
parallax used for?
• to measure a star’s temperature
• to measure a star’s distance from
Earth
• to measure a star’s mass
• to measure a star’s magnitude
๏parallax measures a star’s distance
from earth
Review#2 What does
a light year measure?
• temperature
• time
• distance
• magnitude
๏distance
Reviw#3 An AU is
used for distances
between...
• stars
• galaxies
• solar systems
• planets
๏planets
Review#4 Which
measurement is used
for distant stars?
• AU
• parallax
• inverse square law of light
• light year
๏inverse square law of light