SUN & SEASONS

UFO

Size of Stuff in the Sky

Earth

* Eye/brain gauges angular size

soccer ball

alone doesn’t giveyou true size!

Moon & Sun

Earth

= 0.5o

Ack![Pinky test.]

Angular Size & Distance

Earth

= 0.5o

>

d

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1

SOLAR MOTIONS

APPARENT MOTION OF SUN (Viewed from Earth)

• Diurnal (Daily) Motion: East to West, relative to horizon; rises & sets

• Annual Motion: Eastward relative to stars - reflection of orbit of Earth about sun

- apparent path: Ecliptic

Meridian

Earth’s Orbit1 2

Viewed from Earth,Sun circles sky once per year.

12

Sun appears to passthrough 13

constellations:‘Zodiac’

Gemini

Cancer

Leo

etc . . .

Ophiuchus

EQUINOXES &SOLSTICES

VernalEquinox

AutumnalEquinox

Winter &SummerSolstices

• Vernal Equinox: March 21

• Autumnal Equinox: September 23

• Summer Solstice: June 21

• Winter Solstice: Dec 21

DAILY SOLAR MOTIONCHANGES WITH SEASONS

• Motion N/S of Celestial Equator

> Rising/setting points vary> Noon altitude varies> Duration of daylight varies

• Mar 21 – Sep 23: sun north of celestial equator

sun above horizon > 12 hrs

• Sep 23 – Mar 21: sun south of celestial equator

sun above horizon < 12 hrs

• Sep 23 & Mar 21: sun on celestial equator sun above/below horizon 12 hrs

41o

Mar 21Sep 23

41o

Mar 21Sep 23

90o – 41o

23.5o

[State College]

• Annual variation in motion of sun in sky results from . . .

change in orientation of Earth relative to sun:

SummerSolstice

WinterSolstice

p. 24

Northern Winter Solstice

Northern Summer Solstice [Demo, #3]

REASONSFOR THESEASONS

• Temperature varies from season-to-season – Why?

- Sunlight more direct in summer (than in winter)

- More daylight hours in summer (and fewer in winter)

Northern Winter Solstice

Northern Summer Solstice

Summer Sun

Winter Sun

• No significant temperature change due to change in Earth-sun distance

- e.g., N. hemisphere not much closer to sun than S. hemisphere in summer

- Earth actually closest to sun in January!

JanuaryJuly

Earth’s Orbit – Exaggerated

Earth only ~ 3% closer in January than July