Transcript Motion in the Sky & Getting to know the Sky

This is what it’s all
about…
The Celestial Sphere
Useful concept for:
1. Finding your way
around the sky
(astronomical
coordinate system)
2. Thinking about the
motions of the earth
Your local “zenith” and “meridian”
Great circle that
connects the NCP,
the SCP, and zenith
is the “meridian”
Locally, your
meridian separates
“East” from “West”
Coordinates on the Celestial Sphere
Declination (): analogous to
latitude on the Earth (celestial
equator defines the “zero” of
declination)
Right Ascension ():
analogous to longitude on the
Earth (location of sun on first
day of spring defines the “zero”
of right ascension)
The Celestial Sphere
Regions of the celestial
sphere are divided up
into “Constellations”
Only the very brightest
stars in a handful of
constellations are
indicated here
Constellations - 88 in total
Some constellations are easy to recognize from the patterns of stars. Very
few are easy to imagine as their corresponding image (mythological
character or animal). Orion (the hunter) is one of the exceptions!
Cassiopeia - The Queen (mother to Andromeda); see
Clash of the Titans (they’re all in the sky)
Constellations have well-defined borders
on the sky (set by the International
Astronomical Union in 1928)
Constellations of the Zodiac
(the “sun sign” constellations of astrology)
Called the sun
signs because
they are all
located along
“the ecliptic”, the
path of the sun
through the sky
(= the earth’s
orbit around the
sun).
Over the course
of a year, the sun
(as viewed from
the earth),
“travels” through
each of these
constellations.
Where (approximately) was this picture was taken?
The Celestial Sphere
Which constellations you
can see on any given
night depends on
1. Your latitude (you
can’t see through the
earth!)
2. The time of year (or
“season”)
With the exception of stars that are near the NCP,
astronomical objects “rise” & “set” when seen from Boston
Constellations of the Zodiac
(the “sun sign” constellations of astrology)
Called the sun
signs because
they are all
located along
“the ecliptic”, the
apparent path of
the sun through
the sky (= the
earth’s orbit
around the sun).
Over the course
of a year, the sun
(as viewed from
the earth), the
sun travels
through each of
these
constellations.
Zodiacal Constellations and the Sun
Which constellations could you SEE (i.e., on a given night)?
Hint: if you can see the sun, you can’t see any other stars
Path of the Sun at Different Times of Year: Northern Hemipshere
From March 21-22 (spring equinox) to
September 22-23 (autumnal equinox), the
sun rises and sets north of due east/due
west, so the days are long.
From September 22-23 to March 21-22,
the sun rises and sets south of due
east/due west, so the days are short.
Equniox = equal night
The height of the sun at noon and its
rising/setting point depend upon your
latitude on earth. The poles are truly
extreme (constant day/night for months).
Midnight Sun
Near the earth’s poles, the sun never sets during mid-summer.
You pay for all this summer sun by never seeing the sun in mid-winter!
Tilt of the Earth Relative to the Sun’s Rays
The earth’s equator is
inclined at 23.5o to the
plane of its orbit around
the sun (the “ecliptic”).
This is the true cause of
the seasons.
What season is it in
South America?
What season is it in North
America?
Solar “Irradiance”
The greater is the angle between the surface of the earth and the sun’s rays, the
more power per unit area the surface of the earth receives.
Seasons and the Earth’s Orbit
You experience winter when your hemisphere is pointed away from the direction of the
sun, and summer when your hemisphere is pointed toward the direction of the sun.
Precession of the Equinoxes
Like a spinning top, the earth’s rotation axis
“precesses”, constantly changing the direction of the
North pole with respect to the sky.
This “minor motion” is very slow (takes 26,000 years
to complete), but is important to navigation by the stars
and causes seasons to shift!
Right now, the North Star is “Polaris” (the tail star of
the Little Dipper). Five thousand years ago the North
Star was Thuban, and in 14,000 it will be Vega.