Questions: Seasons What causes the seasons? How do we mark the progression of the seasons? What is the seasonal motion of the sun in the sky? What could cause the seasonal motion of the sun to change over time? Reading: Chapter 2.3 What causes the seasons? John, who lives in Canada, claims that in June it is summer in Canada because the sun is closest to the Earth at this time. Maya, who lives in Australia claims that in December it is summer in Australia because the sun is closest to the Earth at this time. How to resolve this contradiction? 1
What causes the seasons? Seasons are opposite in the N and S hemispheres, so distance cannot be the reason. The real reason for seasons involves Earth s axis tilt. What causes the seasons? Direct light causes more heating. Seasons depend on the orbit (revolution) of the earth around the sun and how the tilt of the Earth s axis affects the directness of sunlight and length of days 2
What causes the seasons? The tilt of the earths rotation axis relative to the plane of its orbit remains constant, so its orientation relative to the sun changes. What causes the seasons? This means that for half the year the Northern Hemisphere is tilted more towards the sun and for the other half of the year the Southern Hemisphere is tilted more towards the sun. 3
What causes the seasons? The Hemisphere receiving more direct sunlight will experience summer, the other Hemisphere, winter. Marking the progression of seasons We define four special points: summer solstice: June 21 winter solstice: December 21 spring (vernal) equinox: March 21 fall (autumnal) equinox: September 22 4
How do we mark the progression of the seasons? Summer solstice: June 21 Day when Northern Hemisphere is maximally tilted towards the sun and gets most direct sun. How do we mark the progression of the seasons? Summer solstice: June 21 The Sun appears to be directly overhead at noon for places situated at latitude 23.44 degrees north, known as the Tropic of Cancer. (Why?) 5
How do we mark the progression of the seasons? Summer solstice: June 21 Places situated at latitude 66.56 degrees north, known as the arctic circle will see the Sun just on the horizon during midnight, and all places north of it will see the Sun above horizon at any time of the day. How do we mark the progression of the seasons? Summer solstice: June 21 Places situated at latitude 66.56 degrees south, known as the antarctic circle will see the Sun just on the horizon during midday, and all places south of it will not see the Sun above horizon at any time of the day. 6
How do we mark the progression of the seasons? Winter solstice: December 21 Day when Southern Hemisphere is maximally tilted towards the sun and gets most direct sun. How do we mark the progression of the seasons? Winter solstice: December 21 The Sun appears to be directly overhead at noon for places situated at latitude 23.44 degrees south, known as the Tropic of Capricorn. (Why?) 7
How do we mark the progression of the seasons? Winter solstice: December 21 Places situated at latitude 66.56 degrees south, known as the antarctic circle, will see the Sun just on the horizon during midnight, and all places south of it will see the Sun above horizon at any time of the day. How do we mark the progression of the seasons? Winter solstice: December 21 Places situated at latitude 66.56 degrees north, known as the arctic circle will see the Sun just on the horizon during midday, and all places north of it will not see the Sun above horizon at any time of the day 8
Marking the progression of seasons Spring (vernal) equinox: March 21 Fall (autumnal) equinox: September 22 Times when neither Hemisphere is more tilted towards the sun. Both hemispheres get same direct sunlight. Marking the progression of seasons Spring (vernal) equinox: March 21 Fall (autumnal) equinox: September 22 The Spring equinox marks the transition from the Southern Hemiphere being tipped towards the sun to the Northern Hemiphere being tipped towards the sun. 9
Marking the progression of seasons Spring (vernal) equinox: March 21 Fall (autumnal) equinox: September 22 The Fall equinox marks the transition from the Northern Hemisphere being tipped towards the sun to the Southern Hemisphere being tipped towards the sun Marking the progression of seasons Spring (vernal) equinox: March 21 Fall (autumnal) equinox: September 22 On the fall and spring equinox, where would you expect to see the sun overhead at noon? 10
Summer Days. Although the Northern Hemisphere gets its most direct sunlight on the summer solstice in June, the hottest days of summer are in July/August. Summer Days. Although the Northern Hemisphere gets its most direct sunlight on the summer solstice in June, the hottest days of summer are in July/August. This is because it takes time to heat up the earth and oceans. Can you explain the coldest months of winter similarly? 11
The tilt of the Earth s axis also explains the length of days and nights On the equinoxes, day and night are roughly of equal length. Between the Spring equinox and the Summer solstice, days in the Northern Hemisphere grow longer as it is tilted towards the sun, and hence this hemisphere gets more energy The opposite is true in the Southern Hemisphere. Summer Days. On the summer solstice, places in the Northern Hemisphere have their longest day (vice versa in the Southern Hemisphere). Between the Summer solstice and the Fall Equinox, days in the Northern Hemisphere grow shorter as its tilt towards the sun decreases, and hence this hemisphere gets less energy. The opposite is true in the Southern Hemisphere. Summer Days. 12
Based on the previous slide can you explain the length of days and nights from the Fall equinox to the Winter solstice to the Spring equinox? Winter Wonderland. High Noon Because of the tilt of the earth s axis, the apparent path of the sun (ecliptic) is tilted at an angle of 23.5 degrees with respect to the celestial equator. This tilt causes the sun to appear to follow different paths in the sky in summer and winter. 13
High Noon We can recognize solstices and equinoxes by Sun s path across sky: Summer solstice: Highest path, rise and set at most extreme north of due east. Winter solstice: Lowest path, rise and set at most extreme south of due east. Equinoxes: Sun rises precisely due east and sets precisely due west. The Ecliptic and the Equinox The apparent path of the sun (ecliptic) is tilted at an angle of 23.5 degrees with respect to the celestial equator. Hence the ecliptic crosses the celestial equator twice a year The crossing points mark the Spring Equinox and Fall Equinox. 14
The Zodiac Because of its relationship to changing seasons, ancient cultures attached great significance to the ecliptic. It marks the center line of the zodiac ( circle of animals ), a band of the sky divided into 12 parts of width 30 degrees. The signs of the zodiac are no longer of special importance in astronomy. The Planets The planets can be seen along the ecliptic appearing like bright stars. Why are they seen along the ecliptic? 15
The Planets As Venus and Mercury orbit inside Earth s orbit, they never get far from the sun and are visible in the west after sunset or in the east before sunrise. Venus can be very bright, but Mercury is difficult to see near the horizon. The Planets By tradition, any planet in the sunset sky is called an evening star. Any planet in the dawn sky is called a morning star. Venus seasonally switches from morning to evening star 16
Precession of Earth s axis Precession may affect the severity of the different seasons and may cause extreme climate changes such as ice ages Earth s axis precesses like the axis of a spinning top What causes the seasons? Summary The orbit of the earth and the tilt of the Earth s axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter How do we mark the progression of the seasons? The summer and winter solstices are when the Northern Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight. 17
Summary What is the seasonal motion of the sun in the sky? The tilt of the earth causes the days in the Northern hemisphere to be longer in summer and shorter in winter and vice versa in the Southern Hemisphere. The tilt of the ecliptic causes the sun to appear to follow different paths in the sky in summer and winter What could cause the seasonal motion of the sun to change over time? The tilt remains about 23.5 degrees (so the season pattern is not affected), but Earth has a 26,000 year precession cycle. This precession may cause extreme climate changes 18