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 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 Earth takes one year to make a complete orbit around the Sun. The plane of the ecliptic is an imaginary planar surface in space containing the Earth’s orbit and the Sun: Earth Sun The Earth takes one year to make a complete orbit around the Sun.

(a) the Earth’s orbit around the Sun The Earth’s orbit is an ellipse. Ellipses can have various shapes, from: Ellipses are sometimes called ovals “squashed”, or more technically, “orbits of high eccentricity”

to: nearly or completely circular, or more technically, “orbits of low or zero eccentricity” Circles are special cases of ellipses.

Ellipses are characterized by their eccentricity e, which varies from:

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 108 km We talk about 10 to the power eight, or ten to the eight where the 108 means that you need to multiply 1.49597900 by 10 eight times! * pretty impressive accuracy!

It’s defined as one Astronomical Unit, i.e. 1 AU= 1.49597900 x 108 km 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 108 km 1 AU For example, Mercury is only approx. .4 of an Astronomical Unit from the Sun, whereas Pluto is 100 times that far away on average! We’ll find Astronomical Units (AUs) convenient when we compare distances between the Sun & other planets in our Solar System.

3.34 x 106 km during the course of a year. The small eccentricity (0.0167) of the Earth’s orbit means that its distance from the Sun varies by 3.34 x 106 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. The Earth at its perihelion, that is, its closest point to the Sun, at the start of January. It is at aphelion, its furthest point from the Sun, in early July.

(b) The origin of the seasons on 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. Earth 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. Seasons on Earth are very important to lifecycles - it is reasonable to expect that life on Earth could be quite different without them.

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? Give this question some thought, then write down an answer before continuing on to the next slide!

Here are some possible answers:

It’s warmer in summer than in winter because ... (a) the Earth is closer to the Sun then, (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) the tilt of the Earth’s axis means that the hemisphere experiencing summer gets more concentrated sunlight than does the hemisphere experiencing winter, It’s worth checking out each of these responses - even ones you would not have chosen. (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...

Did you investigate alternatives (c) and (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. What causes both these phenomena?

… both these effects are due to the tilt of the Earth’s rotational axis: 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 autumn Northern summer Northern winter Southern summer Southern winter Northern autumn Southern spring

(c) The Earth’s precession The direction of the rotational axis of the Earth is not fixed in space. We’ll discuss tidal forces in more detail when we study the Moon. Tidal forces due to the Sun & Moon cause it to very slowly rotate: 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. This could cause glaciers to return one day to northern European landmasses!

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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This is a very common response. (a) “It’s warmer in summer than in winter, because the Earth is closer to the Sun then.” This is a very common response. Most people answer the question this way.

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?

Back to the alternative answers 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 Remember that the Earth is closest to the Sun in January, in the Southern Summer, but furthest from the Sun in July, in the Northern Summer

Back to the alternative answers

(b) The Earth’s rotational axis is tilted by 23½o with respect to a line drawn perpendicular to the plane of the ecliptic. 23 23½o Remember that the plane of the ecliptic is the imaginary surface on which the Earth traces out its orbit around the Sun. Could this tilt make one hemisphere of the Earth sufficiently closer to the Sun compared to the other hemisphere for seasons to result? plane of the ecliptic

Back to the alternative answers 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) ). The eccentricity of the Earth’s orbit makes several thousand times larger a variation than this. Back to the alternative answers

Back to the alternative answers

(c) The Earth’s rotational axis is tilted by 23½o with respect to the plane of the ecliptic. 23 23½o Remember that the plane of the ecliptic is the imaginary surface on which the Earth traces out its orbit around the Sun. plane of the ecliptic

If we take the case when the northern hemisphere is in summer ... sunlight When the Sun appears to be higher in the sky, its light strikes the Earth from a higher angle. 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 ... The closer the Sun is to directly overhead, the more concentrated are the rays of sunlight striking the Earth. Concentrated beam of sunlight in Summer

… than in winter, when the Sun is lower in the sky. When the Sun appears to be low in the sky, its rays are spread out as they strike the Earth. Diffuse, “spread-out” beam of sunlight in Winter

… 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. Back to the alternative answers

Back to the alternative answers

(d) The Earth’s rotational axis is tilted by 23½o with respect to the plane of the ecliptic. 23½o Remember that the plane of the ecliptic is the imaginary surface on which the Earth traces out its orbit around the Sun. plane of the ecliptic

If we take the case when the northern hemisphere is in midsummer ... then the north pole has continuous daylight and locations in the northern hemisphere have long periods of daylight, sunlight equator whereas locations in the southern hemisphere have long nights. Remember that the north & south poles are at each ends of the rotation axis (shown here as a solid red line) the south pole is in continuous darkness

Back to the alternative answers 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 Our atmosphere helps the Earth retain heat from day to day.

Back to the alternative answers