Online Astronomy eText: The Planets Using the Planetary Data Table As a Reference in Class       The main purpose of the Data Table is as a reference in class. When I am discussing various topics involving the planets, I find it convenient to refer students to the contents of the Table.       As an example, the semi-major axis of an orbit, which is the number usually used to describe the size of the orbit, is sometimes referred to as "the average distance of the planet from the Sun." The reason for this is that the planet is closest to the Sun (at perihelion) when it is at one end of the major axis of the orbit, and furthest from it (at aphelion) when it is at the other end of the major axis. Taking the average of the minimum and maximum distances involves adding them together and dividing the result by two. But since they represent positions at either end of the major axis, adding them together gives a value equal to the length of the major axis, so dividing that result by two yields half the major axis, or the semi-major axis.       When discussing this in class I show the approximate aphelion and perihelion distances for a few of the planets and the averages of these values, which are of course their semi-major axes. To allow students to see that this arithmetic does work as described, the Data Table shows all three of the numbers involved in the calculation to considerable accuracy. This is the ONLY reason these numbers are shown to this level of detail in the Data Table; for all other purposes, considerably rounded-off values are perfectly adequate. As an Aid in Studying for the Examinations       Now as it happens, one of the things which it is useful to actually know about the planets is the size of their orbits, meaning the semi-major axes just described above. Since this represents their average distance from the Sun, its value substantially affects their weather, planets with smaller orbits having almost without exception warmer surface temperatures than planets with bigger orbits (the only exception, Venus, being due to the runaway greenhouse effect caused by its thick atmosphere of carbon dioxide). In addition, the very way in which the planets were formed is affected by their distances, as planets which formed close to the Sun where it is hot, and was very hot 4.5 billion years ago when the Solar System was forming, can only be made of refractory materials (materials which are hard to vaporize, such as rocks), whereas planets formed far from the Sun, where it is and was cold, can be made of rocky materials AND of volatile materials (materials which vaporize at relatively low temperatures, but could be solid at very low temperatures, such as ices). Because of this, the planets close to the Sun started off as bits of rocky materials and the planets far from the Sun as bits of icy materials (there were also rocks further out, but the atoms which make icy materials happen to be much more abundant than the ones which make rocky materials, so the primary ingredients in planets which formed far from the Sun were ices). Since rocky materials are relatively rare the rocky bodies close to the Sun formed slowly and ended up as fairly small objects, incapable of holding onto the light gases (primarily hydrogen and helium) which made up most of the Solar Nebula from which the planets formed, and by the time they had grown large enough to hold onto heavier gases, those gases had been blown away from the nascent Sun, leaving them as relatively small, entirely rocky objects.       In the outer Solar System, the ices which formed the original cores of the outer planets existed in much larger amounts, so they formed much more quickly, and four of them grew large enough fast enough that they were able to start pulling light gases to themselves at just about the same time the Sun started dispersing those gases, allowing them to acquire a substantial and in some cases very substantial outer layer of hydrogen and helium. This made those planets quite different in size and composition from the inner planets.       Since so many things depend upon where the planets are and were in the Solar System, it is nice for students to be aware of approximately how far the planets are from the Sun, or in other words the semi-major axes of their orbits, and as a result this is specifically asked for as part of the essay question on orbital motions. However, there is no conceivable reason that students in an introductory class should be required to know those distances to the several-digit accuracy shown in the columns which are used to demonstrate the relationship between the perihelion distance, aphelion distance and semi-major axis. And there is no reason at all that the students should know the actual perihelion or aphelion distances. Just knowing the relationships involved, that is the concepts involved, should be quite adequate.       As a result, to emphasize the non-importance of knowing the more accurate numbers, they are all shown in slanted (italicized) numerals save for the perihlion distances of Pluto and Neptune, which are shown in normal figures to emphasize the fact that Pluto, because of its relatively high eccentricity, despite its much larger orbit actually comes closer to the Sun at perihelion than Neptune does. And to show the sort of accuracy which is appropriate for test purposes, the semi-major axes are shown in a second column, which is rounded (save for Mars) to only a single non-zero digit (for Mars, since rounding 1.5 up to 2 or down to 1, would produce a large error, the value is shown to 2 digits). Only this column for semi-major axes is shown in such a crudely rounded-off fashion, but if any of the numbers in the Data Table are actually needed for test purposes they can ALL be rounded off to this low level of accuracy. In general, if rounding off a number to only one non-zero digit produces an error of less than 10 or 15 percent, then I would consider that rounded-off value to be perfectly accurate for all test purposes. As a result, only numbers whose first non-zero digit starts with a 1 or a 2, such as the semi-major axis of Mars, might need to be rounded off to two, instead of one non-zero digit in order to achieve a reasonable level of accuracy.       Aside from the fact that IF you need to know some numbers they can be rounded off considerably, you do NOT need to know all of or even most of the numbers in this table. To see which numbers you need to know, look at the essay questions involving the planets (numbers 1 through 4), to see if any of them specifically ask for some kind of number. If a particular quantity, such as the orbital period or the orbital inclination of a planet is NOT asked for in one of those questions, then you do NOT need to know the numbers involved.       However, although you may not need to know such numbers for many of the quantities shown in the Data Table, there is always a reason the numbers are there, if only for lecture purposes, and a result, you DO need to know what the numbers represent. In addition, in some cases where the numbers are unusual, and I make some specific reference to them, you may at least need to have some vague idea of what "representative" numbers are like.       As an example of that, in discussing the seasons on the planets, if a planet such as Mercury has a rotational tilt close to zero or to 180 degrees it will not have seasons, while if it has a tilt such as that of Mars which is similar to our tilt then it will have seasons at least vaguely like ours, and if it has an "extreme" tilt (meaning, closer to 90 degrees than to zero or 180 degrees) such as Uranus, then it will have extreme seasons. If you had to answer essay question 2, which deals with, among other things, seasons on the other planets, you would need to be aware of these three distinct possibilities, and which planets have what sort of tilts, or at least know an example or two for each tilt. You wouldn't need to know the actual values of the rotational tilts, but you would need to have an approximate idea of what the numbers represent. As A Convenient Way to Find Information That You Might Need       Even if you didn't have the Planetary Data Table as a reference, you'd need to know some of the numbers or approximate numbers, or at least vague concepts involving those numbers, if you want to know anything about what the planets are like. But if you didn't have the Data Table you'd have to hunt through several chapters or appendices in the textbook in order to find the numbers, and see what they are like. So having the Data Table available online should make it easier to find things, whether you need to know them for test purposes or not. Printing the Planetary Data Table      If you try to print the Planetary Data Table, the web page version may or may not print properly. If that is the case, and you have some need to print the Table, use the PDF version (and Page Scaling, if needed).