Sep 13 - Seasons. Precession

Question 1

What causes the seasons?

Answer 1

• The tilt of the Earth’s axis causes sunlight to fall differently on Earth at different times of the year
• NOT dependent on the Earth’s distance from the Sun

Question 2

With no axis tilt…

Answer 2

= earth has no seasons

Question 3

Where does Earth tilt towards?

Answer 3

Polaris; as a result, the orientation of the axis relative to the sun changes over the course of each orbit, and the different hemispheres experience opposite seasons

Question 4

Sun on the Northern Hemisphere:

Answer 4

tipped toward sun in June, away in December

Question 5

Sun in the Southern hemisphere:

Answer 5

tipped away from sun in June, towards in December

Question 6

When tipped more towards the sun…

Answer 6

Experience harsher/concentrated sunlight and shorter shadows

Also allows the sun to follow a longer and higher path through the sky - giving more hours of daylight

Question 7

When tipped away from sun…

Answer 7

Experience less concentrated sunlight and longer shadows
Shorter path for the sun

Question 8

Both hemispheres have equal sunlight during…

Answer 8

…the equinoxes

• Thus, spring for the hemisphere on the way from winter to summer (Northern), and fall for the hemisphere on the way from summer to winter (Southern)

Question 9

June Solstice:

Answer 9

summer solstice, moment where the northern hemisphere is tipped most directly towards the sun and receives the most direct sunlight

Question 10

December Solstice

Answer 10

winter solstice, moment where the northern hemisphere receives the least direct sunlight

Question 11

March Equinox

Answer 11

spring equinox, when the Northern Hemisphere goes from being tipped slightly away from the sun to being tipped slightly towards the sun

Question 12

September Equinox

Answer 12

fall equinox, occurs sept. 22nd and is the moment when the NH first starts to be tipped away from the Sun

Question 13

We typically say that each equinox/solstice marks the first day of a season. However, this is when the sun reaches its maximum tilt towards the sun - so why do we deem this as the beginning of the season, rather than the middle?

Answer 13

11) Was easier for ancient people to identify the days on which the sun reached extreme positions in the sky (highest/lowest point)

2) We usually think of the seasons in terms of weather, and the warmest summer weather tends to come 1-2 months after the solstice (EX: like putting soup on a stove, it takes a while to fully warm up - like how it takes time for sun to heat the grounds and oceans)

Question 14

Seasons around the world:

Answer 14

High latitudes have more extreme seasons
- Most extreme at North/South poles

Differ in equatorial regions, as the equator gets its most direct sunlight on the two equinoxes and least direct on solstices

Question 15

Why Southern Hemisphere seasons are milder on earth

Answer 15

NH seasons are slightly more extreme than those of the SH - WHY?

Most land lies across the NH, and more ocean in the SH

In this way, the bodies of water moderate SH climate & NH land and less ocean heats up/cools down more easily

Question 16

How does the orientation of Earth’s axis change with time?

Answer 16

Precession: gradual wobble that alters the orientation of Earth’s axis in space

• Doesn’t change the amount of axis tilt - thus, not the pattern of the seasons
• Changes the points in Earth’s orbit at which the solstices and equinoxes occur, and therefore changes the constellations that we see at those times.

Question 17

How does precession occur?

Answer 17

Caused by gravity’s effect on a tilted, rotating object

• EX: trying to balance a bayblade - falls to one side inevitably, but stays up when spinning rapidly (law of conservation of angular momentum)

Question 18

Precession & Earth:

Answer 18

§ Earth is not quite a perfect sphere, because it bulges at its equator. Because the equator is tilted 2312° to the ecliptic plane, the gravitational attractions of the Sun and Moon try to pull the equatorial bulge into the ecliptic plane, effectively trying to “straighten out” Earth’s axis tilt.
However, like the spinning top, Earth tends to keep rotating around the same axis. Gravity therefore does not succeed in straightening out Earth’s axis tilt and instead only makes the axis precess.