Astronomy 1 (Lecture Class) InformationAdditional Notes for Essay Question 2 The following shows notes sent by a student in preparation for answering essay question 2, about the rotation of the sky and its various effects. He/she wanted me to comment on the notes, so he/she could get a better idea of what I was looking for on the exam. I don't usually have the time to comment in as much detail as I did in this case and having done so, decided to post the student's notes and my reply (in boldface), as a guide for other students. For reference, the current version of the question has been added to the start. Question 2:Describe the apparent motions of stars in different parts of the sky, as seen at different latitudes. Discuss how the Sun's motion differs from that of the stars, and how its changing position causes the seasons. Discuss how the seasons differ at different latitudes, and discuss other temperature/weather variations which depend upon location, such as climatic zone and geographical effects. Explain how the tilts of the planets' axes of rotation affect the intensity of their seasons at different latitudes. Compare/contrast orbital effects with effects caused by the rotations of the planets. Student's Notes and My Comments The altitude of the celestial pole is the same as your latitude. At the poles the celestial pole is overhead and some stars are always up and others always down. Half the sky is always up and half always down. A good, quick summary, but you should indicate why stars are always up/down (they are circling parallel to the horizon, because that's where the Celestial Equator is). At the equator the celestial pole is on the equator and half of the stars are always up and half are always down. The Pole is on the Horizon, not the Equator (I understood what you meant, but the wording is incorrect); and again, you need to explain why they are up/down as they are (since the CE is vertical, all paths are cut in half by the Horizon; a diagram would help with this topic. At mid-latitudes the celestial pole is at an angle to the horizon and some of the stars are always up and some are always down and some of the stars are up half of the time and others are down half of the time. As presented here, you could get away without specifying the angle, since you indicated it was the same as the latitude at the start; but it wouldn't hurt to repeat the relationship, or to note that for us, it's about 34 degrees. Also, for the best grade you'd want to explain in detail which stars are up longer or less, with or without a diagram according to your ability to describe it better with words or diagrams. At the very least you'll need to do this for the Sun later in the answer, and if you explain it in detail here you can briefly refer to the earlier explanation when you get to the Sun. That sort of approach tends to indicate actual understanding of the concept, rather than parroting what you've read or heard (not that I think you're doing that; I'm just saying that things that show intelligent understanding are more impressive than things that could be learned by mere memorization). The path that the sun takes around the celestial sphere yearly is called the ecliptic. The inclination of the ecliptic is the reason the sun moves North and South in the sky as the seasons change. The seasons are caused by the 23 degree tilt of the Earth's axis. I'd have worded this as The inclination of the Ecliptic is etc, then The inclination is caused by the 23 degree tilt of the Earth's axis, relative to the Pole of the Ecliptic, or some such thing. Nothing wrong with your statement, just would prefer a more direct statement of HOW the tilt causes the seasons. The earth travels around the sun and in June the Northern Hemisphere leans into the sun and is more directly illuminated for up to a 15 hr. day heating the ground and causing warmer weather. In December the Southern Hemisphere leans into the Sun and the Northern Hemisphere leans away and summer occurs in the Southern Hemisphere and Winter in the Northern Hemisphere. Either need a diagram here, or a very clear explanation of what you mean by "leans". In some ways this sounds like you're saying that the Earth leans one way at some times, and a different way at other times; and of course, it always leans the same way. So you'd want to state this in a way that makes it clear that when we're on one side of the Sun, our tilt causes the N hemisphere to tilt/lean toward the Sun, but on the other side, the same tilt, combined with the Sun's opposite direction, causes the other hemisphere to tilt/lean toward the Sun. In September and March the earth leans sideways, neither towards nor away and both hemispheres are equally favored by sunshine and Spring occurs. The discussion above states the existence of summer/winter more or less adequately, but doesn't explain exactly how the tilt/leaning causes the seasons (namely, the Sun is up higher and longer, or at least higher or longer, during 'summer', and it is up lower and less, or at less lower or less, during 'winter'). It also doesn't compare/contrast the seasons at the Poles or Equator to each other, or to our seasons. A detailed discussion of the nature of the seasons at different latitudes will earn a higher grade than a brief discussion. There are two reasons that it becomes warmer in the northern hemisphere, as the Sun moves northward. It is up longer, because stars closer to the pole stay up longer than stars closer to the equator, and it is up higher. The latter effect mimics climatic zone effects, so that it is sometimes confused with those; but the former only occurs as a result of seasonal effects. The paragraph above addresses the first item mentioned in my previous comment (if this were an actual test, I'd go back and delete the statement, but I don't bother with that in email responses), but it still doesn't compare the seasons at different latitudes. Geographic differences -- on the Earth, which has both liquid (ocean/lake) and solid (continent/land) surfaces, temperatures vary less closer to water and more further from water, so that near the coast temperatures are lower during the day than inland and higher at night than inland. No other planet has such effects, because no other planet has both liquid and solid surfaces. I'd move this to the discussion below, where you need to compare seasons/etc to eccentricity, so that you can point out that the geographic differences are twice as important as eccentricity, for the Earth, with a more direct comparison. How is temperature of a planet affected by the semi-major axis and eccentricity of an orbit? The most obvious effect is that the closer the planet is to the Sun (the smaller its semi-major axis), the warmer it should be. If the eccentricity is small, as it is for the Earth (and all but three other planets) then the weather is hardly affected by the change in distance during one orbit. For the Earth it is coldest in January when we are closest to the Sun and warmest in July. The paragraph below is nearly the same as the first sentence, above. Try not to repeat yourself. On the first exam, you'll have plenty of time, but on subsequent exams, you'll have about the same amount of time, and more questions to answer, so you'll need to manage your time, by presenting your answer as quickly and efficiently as possible (while still maintaining the highest level of detail possible, under the circumstances). The relationship between semi-major axis and average temperatures: Semi-major axis of a planet's orbit (which represents the average distance of the planet from the Sun) is the single most important factor in the average temperature and weather. Almost without exception, planets which are closer to the Sun are warmer than those which are further away. Climatic Zones: The amount of sunshine which is received per square foot of ground depends upon how high the Sun is in the sky. When the Sun is 30 degrees above the horizon, each square foot of ground receives only half a square foot of sunlight. When sunlight shines from nearly overhead one square foot of sunlight falls on one square foot of ground. When it shines at a shallow angle each square foot of sunlight spreads out over many feet of ground. Regions near the poles, even when the Sun is up all the time, receive only a fraction of the sunlight and heat per day that equatorial regions receive in just the few hours when the Sun is nearly overhead. And as a result those regions that are near the equator on any planet where the Sun is normally very high in the sky, and its light is not spread out are relatively warm or tropical whereas those regions which are near the poles, where the Sun is low in the sky and its light is considerably spread out, are relatively cold, or arctic. Very well done paragraph; but I'd move it up, to the area where you discuss the seasons on the Earth. There, you stated that seasons are related to the time the Sun is up, and how high it is. Since its average height is the cause of climatic zones, putting this discussion closer to the discussion of seasons might allow you to cover things in more detail without having to repeat yourself any more than necessary. FINAL NOTES: The following topics were not covered in the notes you sent. (1) Seasons on planets with no tilt (and why those planets don't have seasons), seasons on planets like the Earth, and seasons on planets with extreme tilt. Should have a discussion of each type, with a brief explanation of why no tilt = no seasons, etc, and one example of a planet other than the Earth, which has a particular type of tilt. (2) In discussing eccentricity, although you state that most planets don't have effects due to it, you don't discuss the effects that are observed, on Mars, Mercury and Pluto. Even a C paper should give at least a brief example of how things are different for one of these planets, and an A paper would (usually) discuss each of them, albeit again, probably only briefly, given all the other topics which need to be covered. All in all, you've done a good job with this. There are several places where you could use far more detail, and a couple of places, as noted, where you skipped an important topic; but this would probably earn a passing grade, even 'as is', and with a little more detail (probably in your mind when you sent the note, but left out because it would be too hard to put in an email), could easily earn a B. An A would require a more thorough and complete discussion where indicated, but you don't need to cover every part of the question in excruciating detail to get an A. That just requires that every part is done 'well', and most/several parts are done 'very well'/'exceptionally well'. Even A+ papers don't always discuss every part exceptionally well.