Nearby Constellations

(2-D “Constellations”,from Stars in 3-D !)

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.)

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

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

Christopher Clavius

Pope Gregory XIII The lunar crater Clavius,

with peripheral craters

(Need “Leap Day” Corrections!)

“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”!