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Module 2: Star Gazing Activity 2: Earths Seasons.

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1 Module 2: Star Gazing Activity 2: Earths Seasons

2 Summary: In this Activity, we will investigate: (a) the Earths orbit around the Sun, (b) the origin of the seasons on Earth, and (c) the Earths precession.

3 The Ecliptic At the end of the last Activity we introduced the ecliptic, which is the apparent path of the Sun across the sky. Remember that the plane of the ecliptic is an imaginary surface in space containing the Earths orbit around the Sun. The Earth takes one year to make a complete orbit around the Sun. What else do you know about the Earths orbit around the Sun besides how long it takes?

4 You might think that the Earth travels around the Sun in a circular orbit... (a) The Earths orbit around the Sun You can think of an ellipse as a squashed circle (but note that this one is quite exaggerated!). … but actually the shape of the Earths orbit is an ellipse.

5 squashed, or more technically, orbits of high eccentricity Elliptic orbits can have various shapes, from: to: nearly or completely circular, or more technically, orbits of low or zero eccentricity.

6 Ellipses are characterised by their eccentricity e, which varies from: e = 0 e = e 0.8 A circle is in fact a special case of an ellipse. Circles are ellipses with zero eccentricity.

7 The Earths orbit is nearly circular, with e = Its average distance from the Sun is 149,597,900 km. We write large numbers like this in a mathematical shorthand called scientific notation, where 149,597,900 km = x 10 8 km where the 10 8 is 1 followed by 8 zeros. This is the same as multiplying by 10 eight times.

8 In astronomy, however, we have more convenient way of representing the average distance from the Earth to the Sun: its defined as one Astronomical Unit, where 1 AU = x 10 8 km Well find Astronomical Units (AU) convenient when we compare distances between the Sun and other planets in our Solar System. 1 AU

9 The small eccentricity of the Earths orbit (0.0167) means that its distance from the Sun varies by x 2 x x 10 8 km, or about 5.00 x 10 6 km during the course of a year. * * The factor of 2 is here is because the eccentric orbit can take the Earth to a distance of 1AU AU This is a variation of only about 3% in the overall orbital radius, but represents a distance of about 400 times the Earths diameter. For more information about elliptic orbits, click here

10 (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. The Earth The Sun

11 However, we cant really feel that the Earth is orbiting the Sun (even though the Earth is travelling 30 km/s!), and these days not many people take notice of the Earths orbital position (that is, people dont take much notice of the changing of the constellations in the night sky). Other planets have seasons too. Investigating the reasons for Earths seasons will help us understand the conditions on other planets also. We primarily notice the passing of a year by the cycle of the seasons.

12 So what is the cause of the seasons? Before going on to the next slide, have a think about it yourself: what is the cause of the seasons? Clearly the seasons have something to do with the Earths orbit around the Sun. Yet many people are confused about why the Earth has seasons.

13 The seasons are caused by the changing distance between the Earth and the Sun, and it is warmer in summer because the Earth is closer to the Sun at summer time. This is a very common response - and it is true that the Earth-Sun distance does charge. As we just saw, the Earths distance from the Sun varies by about 3% during its orbit. So could summer occur when the Earth is closest to the Sun? The problem with this idea is that when its summer in the northern hemisphere, its winter in the southern hemisphere, and vice versa. So if this were the correct answer, it would be the same season in both hemispheres at the same time - which is not the case.

14 The Earths rotational axis is tilted by 23.5° with respect to a line drawn perpendicular to the plane of the ecliptic. 23.5° The seasons result from this tilt of the Earths axis of rotation. plane of the ecliptic Earths rotation axis This is true not only of the Earth, but all other planets with tilted rotation axes, as we shall see.

15 The direction of the rotational axis stays (nearly) fixed in space while the Earth orbits the Sun and the hemisphere that seems to lean into the Sun experiences summer, while the hemisphere that leans away from the Sun experiences winter. Thus the tilt of the Earths rotation axis naturally explains why the seasons are opposite in the northern and southern hemispheres. southern hemisphere leans into Sun summer in SH summer in NH (leans into Sun) northern hemisphere leans away from Sun winter in NH winter in SH (leans away from Sun) And six months later the opposite is true:

16 In December, when the southern hemisphere is tilted towards the Sun, the southern part of the Earth receives more sunlight and experiences long summer days. At the same time, the northern hemisphere is tilted away from the Sun and receives less sunlight, experiencing short winter days. equator N S Sun If you live near the Equator, there is not much difference between the seasons all year round.

17 If we take the case when the northern hemisphere is in summer... equator rotation axis sunlight not only does the northern hemisphere receive more sunlight, but it receives more direct sunlight because the Sun is higher in the daytime sky. This helps heat the atmosphere in summer.

18 When the Sun is higher in the summer sky, the sunlight is more concentrated …. Concentrated beam of Summer sunlight … than in winter, when the Sun is lower in the sky, and the sunlight is more diffuse. Diffuse, spread-out beam of Winter sunlight

19 … 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. During spring and autumn, the two hemispheres receive approximately equal amounts of sunlight.

20 northern hemisphere winter southern hemisphere summer northern spring southern autumn northern summer southern winter northern autumn southern spring Lets have a look how it works during the course of a year:

21 As weve already mentioned, if you live near the Equator, you dont really notice the changing seasons during the course of a year. Generally you just have two seasons: dry and wet! If you are at the North or South Pole, then also experience two very long seasons: in summer, the South Pole is leaning towards the Sun and there is daylight for nearly six months. In winter, the South Pole is leaning away from the Sun and it is nighttime for nearly six months.

22 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

23 23.5° plane of the ecliptic So in summary, the cause of the seasons is the tilt of the Earths rotational axis, and as a consequence of this tilt: the Sun is higher in the sky for longer during summer days (and lower in the sky for a shorter number of hours on winter days); and because the Sun in higher in the summer sky, the heating of the Earths surface is more direct (whereas the low winter Suns heating is more diffuse).

24 As we will see, whether the rotational axis is tilted or not determines whether other planets experience seasons too.

25 (c) The Earths precession 23.5° plane of the ecliptic During its yearly orbit around the Sun, the Earths rotation axis is fixed in space. However, if we could watch the orientation of the Earths rotation axis over a very long period of time (about 26,000 years!), we would see that it in fact precesses.

26 The precession of the Earth is due to the gravitational tug of war on the Earth by the Sun and the Moon. The Earths tilt means the Sun and Moon are not aligned with the equator, and both the Sun and Moon try to pulls the Earths equatorial bulge closer to it. The combined pull of the Sun and Moon, along with the Earths own rotation, result in the observed precession. The Earths rotation creates an equatorial bulge (meaning the Earth is fatter at the equator than at the poles). Not to scale!

27 Over a period of 26,000 years, the Earths rotational axis precesses through a complete cycle. Click here to see an animation of precession. 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. * See the previous Activity, Star Patterns *

28 Precession also changes the locations at which seasons occur in the Earths orbit. The Earth is currently 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.

29 Image Credits NASA: View of the Mid-Pacific Ocean

30 Now return to the Module home page, and read more about the Earths seasons and precession in the Textbook Readings. Hit the Esc key (escape) to return to the Module 2 Home Page


32 The Cartesian (i.e (x,y) coordinates) equation for an ellipse is given by: Elliptic Orbits x y where a is the semi-major axis and b is the semi-minor axis. a b If a = b, then the ellipse becomes a circle. The larger a is than b, the more squashed the ellipse is.

33 We can also write the equation for an ellipse in polar coordinates (i.e (r, ) coordinates): x y r where r is the radius and is the angle (from the x-axis). The eccentricity e, which is given by: describes how squashed the ellipse is, with circles having e = 0. r and are measured from the focus of the ellipse.

34 If we use polar coordinates to plot the ellipse, we need to run from = 0 to = 2 (or = 0 to 360°). For a circle, the radius remains constant with angle, and hence our Cartesian plot: x y will look pretty boring in polar coordinates: r An ellipse, however, will look like a sine wave: x y Cartesian r Polar

35 Return to Activity


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