Astronomy 100 -- Apparent Motion of the Sun & Seasons

ANNUAL PATH OF THE SUN ON THE SKY
===================================

Earth Spin-Axis Tilt = 23.5°

Because of this tilt, the ANNUAL PATH of the Sun in the sky is NOT along the Celestial Equator. The Sun moves North and South of the C.E. The path of the Sun is known as the ECLIPTIC.
Northernmost Point = 23.5° North of C.E.

Southernmost Point = 23.5° South of C.E

Position of the Sun with respect to the CE occurs on specific dates. For historical reasons, calendars are adjusted such that these positions repeat every year at the same time of year.

March 21 -- VERNAL EQUINOX
Sun crosses the CE moving North.

June 21 -- SUMMER SOLSTICE


Sun is at FARTHEST Point NORTH of the CE.

Sept. 21 -- AUTUMNAL EQUINOX


Sun crosses the CE moving South

Dec. 21 -- WINTER SOLSTICE


Sun is at FARTHEST Point SOUTH of the CE.

Fix Figure 2-11

---

THE REASON FOR SEASONS
===========================

*** THE TILT OF THE EARTH = 23.5° ***

Because of this tilt, the annual path of the Sun in the sky is NOT along the Celestial Equator. The Sun moves above and below the C.E. This Annual Path of the Sun is known as the ECLIPTIC.
Fix Figure 2-11

Sun's position changes above and below the Celestial Equator.

Highest Point = 23.5° above C.E.

Lowest Point = 23.5° below C.E.

-----------------------------------------

Altitude of the Midday Sun

For any location on Earth,

Altitude of Sun = Alt. of CE + (or - ) Sun's Height North/South of CE

In El Centrof, Altitude of C.E. = 90° - Latitude = 90° - 32.8° = 57.2°.

On March 21 -- Sun on CE (heading North)
Altitude of Midday Sun = (90° - 32.8°) + 0° = 57.2°

On June 21 -- Sun 23.5° Above CE

Altitude of Midday Sun = (90° - 32.8°) + 23.5° = 80.7°

On September 21 -- Sun on CE (heading South)


Altitude of Midday Sun = (90° - 32.8°) + 0° = 57.2°

On December 21 --Sun 23.5° Below CE

Altitude of Midday Sun = (90° - 32.8°) - 23.5° = 33.7°

-----------------------------------------

Change of Altitude of Sun has 3 EFFECTS:

1. The lower the Midday Altitude of the Sun, the more spread the sunlight.
   (Less sunlight hits each square meter of ground)
   

   The Angle of the Sun's Rays
   How to Heat a House

2. The lower the Midday Altitude of the Sun, the less time the Sun spends above the horizon.
   (Sunlight hours in a day are LESS)
   
   Figure shown in class

3. Lower Midday Altitude of the Sun means more energy is absorbed by the atmosphere. This keeps heat from reaching the ground.

---

This is NOT a reason for seasons:

"The Earth is closer to the Sun in the summer"

This statement is UNTRUE!

Presently, the Earth is closest to the Sun on Jan. 4, during our winter! If seasons depended on the distance to the Sun, when we have summer, the rest of the world should also. That is not the case. Instead, when we have summer, the southern hemisphere has winter, and vice versa.

---

---

Earth & Sky Coordinates

Lunar Phases and Motions

Back to The Front Page.

---