Reasons for the Reasons for the SeasonsSeasons

True or False?True or False?

Seasons are caused by the Seasons are caused by the variation in distance variation in distance between the Sun and the between the Sun and the Earth.Earth.

FALSEFALSE

Seasons Misconception 1Seasons Misconception 1 Many people carry the misconception that Many people carry the misconception that

the seasons are due to the distance of the the seasons are due to the distance of the Earth from the Sun. Earth from the Sun.

Consider the following:Consider the following:

The Earth’s orbit around the Sun is nearly The Earth’s orbit around the Sun is nearly a perfect circle (slightly elliptical). a perfect circle (slightly elliptical).

The Earth is a little bit closer to the Sun in The Earth is a little bit closer to the Sun in JanuaryJanuary

A little bit farther from the Sun in July.A little bit farther from the Sun in July.

Rotation vs. RevolutionRotation vs. Revolution

RotationRotation is the spin of an object on its axis. is the spin of an object on its axis.

The Earth rotates once a day (24 hours)The Earth rotates once a day (24 hours)

RevolutionRevolution is the orbit of one object is the orbit of one object

around another.around another.

The Earth revolves around the Sun everyThe Earth revolves around the Sun every

365.24 days.365.24 days.

Tilt of the Earth’s AxisTilt of the Earth’s Axis

The Earth rotates on The Earth rotates on its axis which is tilted its axis which is tilted 23.5 degrees with 23.5 degrees with respect to the eclipticrespect to the ecliptic. . (the plane of the (the plane of the Earth’s orbit.)Earth’s orbit.)

The sun appears to The sun appears to move across the sky move across the sky 1515oo per hour per hour

Think about how many Think about how many degrees usually degrees usually separate time zones separate time zones (15(15oo))

Apparent Motion: Earth’s orbit: Apparent Motion: Earth’s orbit: SeasonsSeasons

Tilt of the earth on its axis Tilt of the earth on its axis The earth is tilted 23.5ºThe earth is tilted 23.5º

Closest/Farthest From the SunClosest/Farthest From the Sun

PerihelionPerihelion ( (closest to the Sunclosest to the Sun) is ) is around January 5, 2012 when Earth is around January 5, 2012 when Earth is about 91,405,436 miles from the about 91,405,436 miles from the Sun.Sun.

AphelionAphelion ( (furthest from the Sunfurthest from the Sun) is ) is around July 5, 2012 when Earth is around July 5, 2012 when Earth is about 94,511,989 miles from the about 94,511,989 miles from the Sun.Sun.

Earth: Northern and Southern Earth: Northern and Southern HemispheresHemispheres

When it is When it is winterwinter in the in the NorthernNorthern hemispherehemisphere

……it is it is summersummer in the in the

SouthernSouthern hemispherehemisphere..

If the seasons were due to our distance from the Sun then If the seasons were due to our distance from the Sun then both hemispheres would have the same seasons at the both hemispheres would have the same seasons at the

same timesame time..

And we know that’s NOT true. And we know that’s NOT true.

So what causes the seasons?So what causes the seasons?

The tilt of the Earth (towards or away from the Sun) affects:

1.Insolation: the number of hours of incoming solar radiation

2.Angle of Incidence: angle at which the sun’s rays hit the earth.

NoteNote: distance still has : distance still has nothingnothing to do with it! to do with it!

Apparent Motion: Earth’s orbit: Apparent Motion: Earth’s orbit: SeasonsSeasons

When your hemisphere is tilted When your hemisphere is tilted towardstowards the Sun, it is the Sun, it is summersummer for for you.you.

Why?Why?Your location will experience Your location will experience

more hours of sunlight at a more hours of sunlight at a higher angle.higher angle.

When your hemisphere is When your hemisphere is pointed pointed away fromaway from the Sun, it is the Sun, it is winterwinter for you. for you.

Why?Why?Your location will experience Your location will experience

fewer hours of sunlight at a lower fewer hours of sunlight at a lower angle.angle.

Check this out!Check this out!

http://astro.unl.edu/naap/motion1/anihttp://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.swfmations/seasons_ecliptic.swf

And this…And this…

http://http://science.sbcc.edu/physics/flash/Lengtscience.sbcc.edu/physics/flash/LengthofDay.swfhofDay.swf

The Earth is curved and this The Earth is curved and this causes differing causes differing angles of angles of

incidenceincidence

Note the relationship between angle and area.

higher angle of incidence = higher intensity = higher temperatures over time

Seasonal Example:Seasonal Example:

On June 21st in the Northern hemisphere:On June 21st in the Northern hemisphere:

1. sunlight strikes the ground 1. sunlight strikes the ground more more verticallyvertically than in December. The radiation than in December. The radiation is is spread out over less groundspread out over less ground and and heats heats the ground more efficientlythe ground more efficiently..

2. the Sun is “out” for a longer period 2. the Sun is “out” for a longer period of time.of time.

Therefore, temperature increases.Therefore, temperature increases.

Summer SolsticeSummer Solstice On the On the summer solstice:summer solstice:

the Sun appears the Sun appears directly overheaddirectly overhead (at 90(at 90oo) to an observer at 23.5 ) to an observer at 23.5 degrees north latitude (degrees north latitude (Tropic of Tropic of Cancer)Cancer)

Winter SolsticeWinter Solstice On the On the winter solsticewinter solstice::

the Sun appears directly overhead the Sun appears directly overhead (90(90oo)to an observer at 23.5 degrees )to an observer at 23.5 degrees south latitude (south latitude (Tropic of CapricornTropic of Capricorn))

SOLSTICESSUMMER, JUNE 21WINTER, DECEMBER 21

Where are the overhead rays of the sun on these days?

Autumnal EquinoxAutumnal Equinox

On the On the autumnal equinoxautumnal equinox:: The Sun will appear directly overhead The Sun will appear directly overhead

at the at the equatorequator..

The Sun The Sun setssets at the North Pole, at the North Pole, ending 6 months of day and beginning ending 6 months of day and beginning 6 months of night.6 months of night.

The Sun The Sun risesrises at the South Pole at the South Pole ending 6 months of night and ending 6 months of night and beginning 6 months of day.beginning 6 months of day.

Vernal EquinoxVernal Equinox

On the On the vernal equinoxvernal equinox:: The Sun will appear directly The Sun will appear directly

overhead at the overhead at the equatorequator..

The Sun The Sun risesrises at the North Pole, at the North Pole, ending 6 months of night and ending 6 months of night and beginning 6 months of day.beginning 6 months of day.

The Sun The Sun setssets at the South Pole at the South Pole ending 6 months of day and ending 6 months of day and beginning 6 months of night.beginning 6 months of night.

EQUINOXESVERNAL, MARCH 21AUTUMNAL, SEPTEMBER 21

Important points :Important points : Seasons are Seasons are NOTNOT due to variation in due to variation in

the Earth-Sun distance. the Earth-Sun distance. We have seasons because We have seasons because Earth’s Earth’s

axis is tilted 23.5 degrees. axis is tilted 23.5 degrees. This tilt causes variation in the This tilt causes variation in the

amount of amount of inincoming coming solsolar radiar radiationation ((insolationinsolation) any particular location ) any particular location receives throughout the course of a receives throughout the course of a year. year.

The tilt also affects the The tilt also affects the angle of angle of incidenceincidence, the angle of the sun’s , the angle of the sun’s rays. rays.