Nearby Constellations
(2-D “Constellations”,
from Stars in 3-D !)
Half-hour time exposure facing north & west.
The stars are tracing counter-clockwise circles,
centered on a point near the prominent
North Star (Polaris).
Notice the Big Dipper at the lower-left.
“Circumpolar”
Stars/Constellations
This several-hour-long time exposure,
taken from tropical northern Australia,
shows the clockwise motion of the
southern stars around the South Celestial Pole.
(Photo by David Miller/DMI.)
Earth shown at the center of the imaginary “Celestial Sphere”
Celestial coordinates: “Declination” (d) and “Right Ascension” (R.A.)
(=> “latitude” & “longitude”!)
The Sun’s apparent path
through the Celestial Sphere . . .
(actually due to Earth’s
annual orbit about the Sun!)
“Summer Constellations” vs. “Winter Constellations”,
due to Earth’s annual revolution about the Sun:
Earth, Sun, and the “Zodiac Constellations” through the Seasons
“Summer Signs” = “Winter Constellations”; “Winter Signs” = “Summer Constellations”
An Earth-bound Observer sees only half the sky at any one time!
“Time Zones” (artificial!), & some famous Meridians around the globe:
“Northern” vs. “Southern”
Celestial Hemispheres
(& Northern/Southern
Constellations!)
(In Northern Winter/
Southern Summer…) =>
What causes the Seasons?
Seasonal observations
Above is for a Northern Hemisphere location
(“Summer” = Higher Sun and Longer Daytimes!)
“Land of the Midnight Sun”? (Or the Noon Darkness…?)
A good view from Stonehenge!
(Sunrise on the Summer Solstice)
Ancient “Observatories”
in the Americas!
(Serving as “Calendars” & “Almanacs”…)
The Sun’s apparent path
through the Celestial Sphere
goes along the “Ecliptic Plane”…
TILTED by ~23.5 with respect to
Earth’s Rotation (NP/SP) Axis!
More Tilt
with respect to the Sun
=> Stronger Seasons!
1 Year  365.2425 Days  365.25 Days
(Need “Leap Day”
Corrections!)
Christopher Clavius
Pope Gregory XIII
The lunar crater Clavius,
with peripheral craters
“Solar Days”
(= a day “by the Sun”)
(= 24 hrs.)
vs.
“Sidereal Days”
(= a day “by the Stars”)
(= 23 hrs., 56 min.)
=> 1 Year  365.25 Solar Days
 366.25 Sidereal Days
“Precession” of Earth’s Axis,
and the temporary nature
of the “North Star” . . .
. . . and the “Vernal Equinox”!