Download 25 August: Getting Oriented, Astronomical Coordinate Systems

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Getting Oriented: Astronomical
Coordinate Systems
• Why are we doing
• Lecture material
for next week’s lab
• Basis for many
(Pyramid of
The basic idea
• Express the position in the sky of an
astronomical object in terms of numbers
• One part of the Greek concept that
there is a mathematical basis to nature
First reference system: the
Horizon System
Two coordinates: altitude and azimuth
Coordinates in the horizon system
• Altitude angle goes from 0d to 90d
• Azimuth angle goes from 0d to 360d
In Horizon System, we see
motions in the sky
• The Sun rises in the east, reaches highest
altitude angle due south, sets in the west
• When the Sun sets, it gets dark and we see
the stars and planets
• The Moon “ “ “ “ “
• The Moon rises at a different time each night
and is seen against a different constellation
• The constellations in the evening sky are
different in different seasons
Fundamental astronomical
The rising and setting of the Sun. Most
people don’t realize other astronomical
objects do this as well
Question for the audience:
What is going on to
cause this east-towest motion of all
objects, rising in
east and setting in
There are other aspects of the night
sky (and daytime sky) that are not
obvious to modern people
Seasonal Variations in the Position of
the Sun (back in horizon system)
This year,
22, 10:09
The changes in the rising (and setting)
locations of the Sun are big
Seasonal differences in the night sky:
go out tonight at
9 PM
• Constellations Bootes in west
• Bright star Vega straight overhead
• Constellation of Scorpius (with bright star
Antares) low in southwest.
• Constellations of Pegasus and Andromeda
just rising
• Check it out with the help of star charts!
By the end of the semester,
the situation will be entirely
There is a phenomenon that could be
called the “Parade of the
Measuring the position of the Sun against
the background stars
The path of the Sun through the stars
The “parade of the constellations”
Obliquity of the Ecliptic and the
Altitude Angle of the Sun
Explanation of Seasonal
Variations: tilt of the Earth’s axis:
obliquity of the ecliptic
Two Lines on
the Sky
•The ecliptic
•The celestial
•See Figure
For new purposes, we need a different
coordinate system
Analogy: I am riding my bike on a dirt road
near Lone Tree, and want to describe to
someone in London the location of a radio
tower I see in the distance.
Question: what system of coordinates do I use?
A New Coordinate System: Celestial
• The stars “stick together” and define
their own reference system. The planets
move with respect to them
• Celestial coordinates are Right
Ascension and Declination
• Right Ascension ….. Longitude
• Declination ….latitude