Presentation on theme: "Star Gazing Activity: Earth’s Seasons. Summary: In this Activity, we will investigate (a) the Earth’s orbit around the Sun, (b) the origin of the seasons."— Presentation transcript:
Summary: In this Activity, we will investigate (a) the Earth’s orbit around the Sun, (b) the origin of the seasons on Earth, and (c) the Earth’s precession.
The Ecliptic The apparent path of the Sun across the sky during the day is called the ecliptic. From our heliocentric point of view, this apparent motion reflects the Earth’s orbit around the Sun.
The plane of the ecliptic is an imaginary planar surface in space containing the Earth’s orbit and the Sun: The Earth takes one year to make a complete orbit around the Sun. Sun Earth
The Earth’s orbit is an ellipse. “squashed”, or more technically, “orbits of high eccentricity” (a) the Earth’s orbit around the Sun Ellipses can have various shapes, from:
to: nearly or completely circular, or more technically, “orbits of low or zero eccentricity”
Ellipses are characterized by their eccentricity e, which varies from: e = 0 e = 1 e 0.8
The Earth’s orbit is nearly circular, with e = 0.0167. Its average distance from the Sun is 149 597 900 km. We usually write awkwardly large numbers like this in a sort of mathematical shorthand called scientific notation, as 1.49597900 x 10 8 km * * pretty impressive accuracy! where the 10 8 means that you need to multiply 1.49597900 by 10 eight times!
We have another even more convenient way of representing the average distance from the Earth to the Sun: It’s defined as one Astronomical Unit, i.e. 1 AU= 1.49597900 x 10 8 km We’ll find Astronomical Units (AUs) convenient when we compare distances between the Sun & other planets in our Solar System. 1 AU
The small eccentricity (0.0167) of the Earth’s orbit means that its distance from the Sun varies by 3.34 x 10 6 km during the course of a year. This is a variation of only about 2% in the overall orbital radius, but it represents a distance of approx. 2 600 times the Earth’s diameter.
(b) The origin of the seasons on Earth Sun Earth As we saw in the last Activity, one (Earth) year is the time it takes for Earth to make a complete orbit around the Sun.
However we primarily notice the passing of a year not by Earth’s orbital position but by the cycle of the seasons. Other planets have seasons too. Investigating the reasons for Earth’s seasons will help us understand the conditions on other planets also.
Most people are confused about why the Earth has seasons. What is your answer to the question: Why is it warmer in summer than in winter?
It’s warmer in summer than in winter because... (a)(a) the Earth is closer to the Sun then, (b)(b) the tilt of the Earth’s axis means that the hemisphere experiencing summer is closer to the Sun than the hemisphere experiencing winter, (c)(c) the tilt of the Earth’s axis means that the hemisphere experiencing summer gets more concentrated sunlight than does the hemisphere experiencing winter, (d)(d) the tilt of the Earth’s axis means that the summer hemisphere has longer hours of sunlight than does the winter hemisphere. Click on alternatives that seems reasonable to see if we agree with you! Then continue...continue
Did you investigate alternatives (c) and (d) ?(c)(d) If so, you will have found that there are 2 major reasons for the seasons on Earth: The differing length of daylight hours in each hemisphere. The amount by which sunlight striking the earth is spread out, depending on whether the sun is high (summer) or low (winter) in the sky.
… both these effects are due to the tilt of the Earth’s rotational axis: 23½ o plane of the ecliptic
Whether the rotational axis is tilted or not determines whether other planets experience seasons too. As the Earth orbits the Sun, we can track the changing of the seasons:
Northern spring Southern spring Southern autumn Northern autumn Northern summerNorthern winter Southern summer Southern winter
(c) The Earth’s precession The direction of the rotational axis of the Earth is not fixed in space. Tidal forces due to the Sun & Moon cause it to very slowly rotate: 23½ o plane of the ecliptic
This is called precession. Over a period of 26 000 years, the earth’s rotational axis “precesses” through a complete cycle.
It is this precession which has gradually shifted the positions of the constellations in the sky, and, in particular, the periods of the year which correspond to each zodiacal constellation. Precession also changes the locations at which seasons occur in the Earth’s orbit. The Earth is now closest to the Sun during southern Summers, but in about 13 000 years it will occur during northern Summers. This may cause southern Summers to become more mild, and northern Winters to become more severe.
Image Credits NASA: View of the Mid-Pacific Ocean http://nssdc.gsfc.nasa.gov/image/planetary/earth/gal_mid-pacific.jpg
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(a) “It’s warmer in summer than in winter, because the Earth is closer to the Sun then.” This is a very common response.
It is true that the Earth is closer to the Sun at one stage of its orbit. As we saw in the last Section, the Earth’s orbital separation from the Sun varies by about 2%, equivalent to about 2 600 Earth diameters. So could summer occur when the Earth is closest to the Sun?
But when its summer in the northern hemisphere, it’s winter in the southern hemisphere - and vice versa. So summer can’t occur at just one particular part of the Earth’s orbit. Back to the alternative answers
The Earth’s rotational axis is tilted by 23½ o with respect to a line drawn perpendicular to the plane of the ecliptic. 23 23½ o Could this tilt make one hemisphere of the Earth sufficiently closer to the Sun compared to the other hemisphere for seasons to result? (b) plane of the ecliptic
The tilt of the Earth’s axis only makes a difference of about 2 000 km - only 1/40 000 th of the variation in the separation of the Earth & Sun. So any distance effects due to the Earth’s tilt will be overwhelmed by the distance effects due to the eccentricity of the Earth’s orbit (which are considered in (a) ).(a) Back to the alternative answers
23 23½ o The Earth’s rotational axis is tilted by 23½ o with respect to the plane of the ecliptic. (c) plane of the ecliptic
sunlight If we take the case when the northern hemisphere is in summer... then the tilt of the Earth’s rotational axis means that the Sun is higher in the sky in daytime in the northern hemisphere, than in the southern hemisphere, which will be in winter...
… and in Summer when the Sun is higher in the sky, each beam of sunlight is spread out less when it hits the ground... Concentrated beam of sunlight in Summer
… than in winter, when the Sun is lower in the sky. Diffuse, “spread-out” beam of sunlight in Winter
Back to the alternative answers … so for the hemisphere experiencing Summer, sunlight striking the Earth is more concentrated and this helps to raise the average temperature. The reverse is true for the hemisphere experiencing Winter... … but this isn’t the only factor causing the Earth’s seasons. Alternative (d) is a contributing factor too.(d)
23½ o The Earth’s rotational axis is tilted by 23½ o with respect to the plane of the ecliptic. (d) plane of the ecliptic
sunlight If we take the case when the northern hemisphere is in midsummer... then the north pole has continuous daylight the south pole is in continuous darkness and locations in the northern hemisphere have long periods of daylight, whereas locations in the southern hemisphere have long nights. equator
Long periods of daylight help to warm the hemisphere experiencing Summer, more than the hemisphere experiencing the short periods of daylight and long cold nights of winter. Back to the alternative answers