Astronomical Coordinates
Summary
• Relative to Observer
– Local Horizon
– Local Meridian
– Local Zenith
• Altitude-Azimuth Coordinates
– Pro: Easy to record for
instantaneous position
– Con: Objects change angles
(coordinates) throughout the
night
– Con: not easy to compare
measurements made by different
observers
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Ancient Alt.-Az. Observatory
• Chankillo, Peru
• 2300 years old
• Marks positions of
sunrise throughout the
year
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Last Class Summary:
Astronomical Coordinates
• Fixed to rotating Earth
– Celestial Equator
– Right Ascension: Measure from Vernal
(March) Equinox
– Declination: measure from Celestial
Equator
Celestial Coordinates
– Pro: stars have fixed coordinates
– Pro: easy to compare measurements made
by different observers
– Note: Sun changes celestial coordinates
throught the year as it follows the ecliptic
through the Zodiac
– Note: Require a clock at each observatory
for coordinate system reference.
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Time of the Seasons
• We have seasons because of the
tilt of the Earth’s axis.
• The Ecliptic appears tilted 23.5°
from the Celestial Equator because
Earth’s axis is tilted 23.5° from the
vertical to the plane of its orbit
• Because of this tilt the Sun appears
half the year North of the Equator
and half the year South of the
Equator
• The seasons are defined based on
the location of the Sun compared to
the Celestial Equator
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Summer
On the first day of Summer:
– The Sun is at its farthest
distance North of the Celestial
Equator (23.5° N declination).
The is known as the Summer
Solstice.
– The Northern Hemisphere is
tilted towards the Sun & we
experience the longest day of
the year (about June 21).
– The Southern Hemisphere is
tilted away from the Sun. This is
the beginning of Winter and the
shortest day of the year there.
– Sun rise and sets at its most
northerly
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Why it’s hotter in the Summer and colder in the Winter
At the summer solstice, the North gets more direct sunlight
and more hours of sunlight. At the same time it is winter in the South
(less direct sunlight, fewer hours of daylight).
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In the winter months the Sun is lower in the sky for
fewer hours and the sunlight is spread over a larger
area.
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“But I thought the reason it was
hotter in the summer was because
the Earth was closer to the Sun in
the Summer.”
• If this were true then why is the Earth
closest to the Sun in January and
farthest from the Sun in July? and
• Why does the Southern Hemisphere
have seasons that are opposite that in
the Northern Hemisphere?
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Winter
On the first day of Winter:
– The Sun is at its farthest South
of the Celestial Equator (23.5° S
declination). This is known as
the Winter Solstice.
– The Northern Hemisphere is
tilted away from the Sun & we
experience the shortest day of
the year (about December 21).
– The Southern Hemisphere is
tilted towards the Sun. This is
the beginning of Summer and is
the longest day of the year there.
– Sun rise and set is at its most
southerly.
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Spring and Autumn
• The Sun crosses the Celestial
Equator heading North on the
first day of Spring (the Vernal
Equinox, about March 21).
• It crosses the Celestial
Equator again heading South
on the first day of Autumn
(the Autumnal Equinox,
about September 21).
• In both the Northern and
Southern Hemisphere days and
nights are of equal length
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Seasons Interactives
• Seasons Interactive I - McGraw Hill
• Seasons Interactive II - University of
Nebraska Lincoln (UNL)
• Sun Motion – UNL
• Seasons Interactive III - Iowa State
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Summary – The Seasons
• The rotation of the Earth on its axis
determines the length of the day
• The orbit of the Earth around the Sun
(revolution) determines the length of the
year
• The tilt of the Earth with respect to the
ecliptic causes the seasons.
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Summary - Equinoxes
• The two equinoxes occur when the Sun crosses the
Celestial Equator
– In March, as the Sun is heading North, and
– In September, as the Sun is heading South
• Equinox means “equal night”
– Length of night is the same as the length of daylight for the
equinoxes
• The Sun will rise due East and set due West.
• For an observer on the Earth’s equator
– The sun will be at the observer’s zenith at local noon.
• For an observer on a pole: the Sun circles the observer on
the horizon…16 hours from half-to-all or half-to-gone.
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Equinoxes
On equator, lat. = 0
At lat. = 20
At lat. = 50
At pole lat. = +90
Summary - Solstices
• The two solstices occur when the Sun reaches its
extreme North or South positions in the sky.
– December Solstice: farthest South
– June Solstice: farthest North
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Solar Altitude Variation
Over Six Months
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Analemma
• Earth's axis is tilted 23.5 degrees
• Earth's orbit is elliptical, not circular
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Analemma Interactive
• http://www.analemma.com/SunGraph/index.html
• APOD - Analemma Movie over New Jersey
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Analemma for Greenwich, England
Latitude +51
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