SUN & SEASONS
Jan 13, 2016
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
d
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