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Astronomy 1010-H
Planetary Astronomy
Fall_2015
Day-3
Course Announcements
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Smartworks Practice: Due Fri. (Aug. 28)
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9 have attempted it, 6 have recorded grades
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Smartworks Chapter 1: Due Wed. (Sep. 2)
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Read Chapter 1 (if you haven’t already)
Read Chapter 2.1, 2.2
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Physics Dept. picnic – TODAY – come by the
office for details.
Definitions & Terms -1
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Orbit: Path a planet takes around its star;
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Path a moon takes around its planet …
Year (yr): A measure of TIME; for this class we will
assume 365.25 days, the time it takes the Earth to
orbit (revolve around) the Sun.
Day (d): A measure of TIME; for this class will will
assume a Solar Day = 24 hours = 86,400 seconds. A
Sidereal Day ≈ 23h 56m
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Second (s): A measure of TIME; the fundamental
time unit of the mks system (of SI).
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Kilogram (kg): a unit of MASS; the fundamental mass
unit of the mks system (of SI).
Definitions & Terms -2
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Kilometer (km): A unit of DISTANCE = 1000 meters
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Meter (m): A unit of DISTANCE; the fundamental
distance of the mks (SI) system.
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Astronomical Unit (AU): A unit of DISTANCE; the
average distance from the Sun to the Earth; ≈1.496 X
108 km
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Light-Year (ly): A unit of DISTANCE = the distance
light will travel in 1 year ≈ 9.5 X 1012 km
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Parsec (pc): A unit of DISTANCE ≈ 3.26 ly = the
distance at which 1 AU subtends 1 arc-second (an
angular measure)
The Universe is Vast!
 Scale:
 The Earth is small,
 Orbits a medium
star,
 In a small group
 On the edge of the
 Virgo Supercluster
 (a smallish cluster)
… which is one of billions of galaxies …
in a universe that
is 13.7 billion
years old.
NASA/ESA/S. Beckwith
(STScI) and the HUDF Team
Light travels at a finite speed:
Solar System
Longer Distances
The Closest Star
Our Galaxy
The Closest
Big Galaxy
The Universe
Measuring Distances
 The universe is vast.
 We need to handle great distances and long times.
 We can do this through the travel time of light.
 Light travels 300,000 km every second.
 We often use times to denote distances. For
example, we may say a friend’s house is two hours
away.
 Astronomy is a time machine!
Measuring Distances
 Close to home (Solar System) we will use:
 Kilometers (for distance to Moon, sizes)
 Astronomical Units (AU) or light-minutes –
distance to most planets
 light-hours for distances to the farther planets
The Inner Solar System
Lecture-Tutorial:
Sun Size – p. 113
 Work with a partner
 Read the instructions and questions carefully
 Talk to each other and discuss your answers with
each another
 Come to a consensus answer you both agree on
 If you get stuck or are not sure of your answer ask
another group
 If you get really stuck or don’t understand what
the Lecture Tutorial is asking as one of us for help