By the end of this section, you will be able to: Show
Recall that the path of an object under the influence of gravity through space is called its orbit, whether that object is a spacecraft, planet, star, or galaxy. An orbit, once determined, allows the future positions of the object to be calculated. Two points in any orbit in our solar system have been given special names. The place where the planet is closest to the Sun (helios in Greek) and moves the fastest is called the perihelion of its orbit, and the place where it is farthest away and moves the most slowly is the aphelion. For the Moon or a satellite orbiting Earth (gee in Greek), the corresponding terms are perigee and apogee. (In this book, we use the word moon for a natural object that goes around a planet and the word satellite to mean a human-made object that revolves around a planet.) Orbits of the PlanetsToday, Newton’s work enables us to calculate and predict the orbits of the planets with marvelous precision. We know eight planets, beginning with Mercury closest to the Sun and extending outward to Neptune. The average orbital data for the planets are summarized in Table 1. (Ceres is the largest of the asteroids, now considered a dwarf planet.) According to Kepler’s laws, Mercury must have the shortest orbital period (88 Earth-days); thus, it has the highest orbital speed, averaging 48 kilometers per second. At the opposite extreme, Neptune has a period of 165 years and an average orbital speed of just 5 kilometers per second. All the planets have orbits of rather low eccentricity. The most eccentric orbit is that of Mercury (0.21); the rest have eccentricities smaller than 0.1. It is fortunate that among the rest, Mars has an eccentricity greater than that of many of the other planets. Otherwise the pre-telescopic observations of Brahe would not have been sufficient for Kepler to deduce that its orbit had the shape of an ellipse rather than a circle. The planetary orbits are also confined close to a common plane, which is near the plane of Earth’s orbit (called the ecliptic). The strange orbit of the dwarf planet Pluto is inclined about 17° to the ecliptic, and that of the dwarf planet Eris (orbiting even farther away from the Sun than Pluto) by 44°, but all the major planets lie within 10° of the common plane of the solar system. You can use an orbital simulator to design your own mini solar system with up to four bodies. Adjust masses, velocities, and positions of the planets, and see what happens to their orbits as a result. Orbits of Asteroids and CometsIn addition to the eight planets, there are many smaller objects in the solar system. Some of these are moons (natural satellites) that orbit all the planets except Mercury and Venus. In addition, there are two classes of smaller objects in heliocentric orbits: asteroids and comets. Both asteroids and comets are believed to be small chunks of material left over from the formation process of the solar system. In general, asteroids have orbits with smaller semimajor axes than do comets (Figure 1). The majority of them lie between 2.2 and 3.3 AU, in the region known as the asteroid belt (see Comets and Asteroids: Debris of the Solar System). As you can see in Table 1, the asteroid belt (represented by its largest member, Ceres) is in the middle of a gap between the orbits of Mars and Jupiter. It is because these two planets are so far apart that stable orbits of small bodies can exist in the region between them.  Figure 1: Solar System Orbits. We see the orbits of typical comets and asteroids compared with those of the planets Mercury, Venus, Earth, Mars, and Jupiter (black circles). Shown in red are three comets: Halley, Kopff, and Encke. In blue are the four largest asteroids: Ceres, Pallas, Vesta, and Hygeia.
Comets generally have orbits of larger size and greater eccentricity than those of the asteroids. Typically, the eccentricity of their orbits is 0.8 or higher. According to Kepler’s second law, therefore, they spend most of their time far from the Sun, moving very slowly. As they approach perihelion, the comets speed up and whip through the inner parts of their orbits more rapidly.
The closest point in a satellite orbit around Earth is its perigee, and the farthest point is its apogee (corresponding to perihelion and aphelion for an orbit around the Sun). The planets follow orbits around the Sun that are nearly circular and in the same plane. Most asteroids are found between Mars and Jupiter in the asteroid belt, whereas comets generally follow orbits of high eccentricity. Glossaryaphelion: the point in its orbit where a planet (or other orbiting object) is farthest from the Sun apogee: the point in its orbit where an Earth satellite is farthest from Earth asteroid belt: the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun perigee: the point in its orbit where an Earth satellite is closest to Earth perihelion: the point in its orbit where a planet (or other orbiting object) is nearest to the Sun satellite: an object that revolves around a planet
Here on Earth, we to end to not give our measurements of time much thought. Unless we’re griping about Time Zones, enjoying the extra day of a Leap Year, or contemplating the rationality of Daylight Savings Time, we tend to take it all for granted. But when you consider the fact that increments like a year are entirely relative, dependent on a specific space and place, you begin to see how time really works. Here on Earth, we consider a year to be 365 days. Unless of course it’s a Leap Year, which takes place every four years (in which it is 366). But the actual definition of a year is the time it takes our planet to complete a single orbit around the Sun. So if you were to put yourself in another frame of reference – say, another planet – a year would work out to something else. Let’s see just how long a year is on the other planets, shall we? A Year On Mercury:To put it simply, Mercury has an orbital period of 88 days (87.969 to be exact), which means a single year is 88 Earth days – or the equivalent of about 0.241 Earth years. But here’s the thing. Because of Mercury’s slow rotation (once every 58.646 days) and its rapid orbital speed (47.362 km/s), one day on Mercury actually works out to 175.96 Earth days. So basically, a single year on Mercury is half as long as a Mercurian (aka. Hermian) day. This is due to Mercury being the closest planet to the Sun, ranging from 46,001,200 km at perihelion to 69,816,900 km at aphelion. At that distance, the planet shoots around the Sun faster than any other in our Solar System and has the shortest year. In the course of a year, Mercury experiences intense variations in surface temperature – ranging from 80 °K (-193.15 °C;-315.67 °F) to 700 °K (426.85 °C; 800.33 °F). However, this is due to the planet’s varying distance from the Sun and its spin, which subjects one side to extended periods of extremely hot temperatures and one side to extended periods of night. Mercury’s low axial tilt (0.034°) and its rapid orbital period means that there really is no seasonal variation on Mercury. Basically, one part of the year is as hellishly hot, or horribly cold, as any other. A Year On Venus:The second closest planet to our Sun, Venus completes a single orbit once ever 224.7 days. This means that a single year on Venus works out to about 0.6152 Earth years. But, once again, things are complicated by the fact that Venus has an unusual rotation period. In fact, Venus takes 243 Earth days to rotate once on its axis – the slowest rotation of any planet – and its rotation is retrograde to its orbital path. Combined with its orbital period, this means that a single solar day on Venus (the time between one sunup to the next) is 117 Earth days. So basically, a single year on Venus is lasts 1.92 Venusian (aka. Cytherean) days. Again, this would make for some confusing time-cycles for any humans trying to make a go of it on Venus! Also, Venus has a very small axial tilt – 3° compared to Earth’s 23.5° – and its proximity to the Sun makes for a much shorter seasonal cycle – 55-58 days compared to Earth’s 90-93 days. Add to that its unusual day-night cycle, variations are very slight. In fact, the temperate on Venus is almost always a brutal 736 K (463 °C ; 865 degrees °F), which is hot enough to melt lead! A Year On Earth:Comparatively speaking, a year on Earth is pretty predictable, which is probably one of the reasons why life is able to thrive here. In short, our planet takes 365.2564 solar days to complete a single orbit of the Sun, which is why we add an extra day to the calendar every four years (i.e. a Leap Year, which 2016 happens to be). But because our axis is tilted, there is considerable variation in the seasons during the course of a year. During the winter, when one hemisphere is pointed away from the Sun, the Sun’s distance from the equator changes by up to 23.5°. As a result, between the summer and winter, the length of days and nights, temperatures, and seasons will go through significant changes.
Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year – up to six months at the North Pole itself, in what is known as a “polar night”. In the southern hemisphere the situation is exactly reversed, with the South Pole experiencing a midnight sun, a day of 24 hours, again reversing with the South Pole. Every six months, the order of this is reversed. A Year On Mars:Mars has one of the highest eccentricities of any planet in the Solar System, ranging from 206,700,000 km at perihelion and 249,200,000 km at aphelion. This large variation and its greater distance from the Sun, leads to a rather long year. Basically, Mars takes the equivalent of 687 (Earth) days to complete a single orbit around the Sun, which works out to to 1.8809 Earth years, or 1 year, 320 days, and 18.2 hours. On the other hand, Mars has a rotation period that is very similar to Earth’s – 24 hours, 39 minutes, and 35.244 seconds. So while the days on Mars are only slightly longer, the seasons are generally twice as long. But this is mitigated by the fact that seasonal changes are far greater on Mars, owing to its eccentricity and greater axial tilt (25.19°). During the winter, the global atmospheric pressure on Mars is 25% lower than during summer. This is due to temperature variations and the complex exchange of carbon dioxide between the Martian dry-ice polar caps and its CO2 atmosphere. As a result, Martian seasons vary greatly in duration than those on Earth, change roughly every six months, and do not start on the same Earth day every Martian year. A Year On Jupiter:Jupiter is another interesting case. Whereas the gas giant only takes 9 hours 55 minutes and 30 seconds to rotate once on its axis, it also takes alson 11.8618 Earth years to complete an orbit around the Sun. This means that a year on Jupiter is not only the equivalent of 4,332.59 Earth days, but 10,475.8 Jovian days. That’s a lot of sunrises! Much like Venus, Jupiter has an axial tilt of only 3 degrees, so there is literally no seasonal variation between the hemispheres. In addition, temperature variations are due to chemical compositions and depths rather than seasonal cycles. So while it does have “seasons”, which change very slowly due to its distance from the Sun – each season lasts 3 years – they are not similar to what terrestrial planets experience. A Year On Saturn:Much like its fellow gas giant Jupiter, Saturn takes it time completing a single orbit of the Sun, but rotates on its axis very rapidly. All told, a year on the planet lasts the equivalent of 10,759 Earth days (or about 29 1?2 years). But since it only takes 10 hours, and 33 minutes to complete a single rotation on its axis, a year on Saturn works out to 24,491.07 Saturnian (aka. Cronian) days. Due to its axial tilt of almost 27 degrees (slightly more than Mars), Saturn experiences some rather long seasonal changes. But due to it being a gas giant, this does not result in variations in temperature. Combined with its distance from the Sun (at an average distance of 1,429.39 million km or 9.5 AU), a single season lasts more than seven years. A Year On Uranus:Uranus has some of the strangest annual and seasonal variations of any planet in the Solar System. For one, the gas/ice giant takes about 84 Earth years (or 30,688.5 Earth days) to rotate once around the Sun. But since the planet takes 17 hours, 14 minutes and 24 seconds to complete a single rotation on its axis, a year on Uranus lasts 42,718 Uranian days. However, this is confounded due to Uranus’ axial tilt, which is inclined at 97.77° towards the Sun. This results in seasonal changes that are quite extreme, and unique to Uranus. In short, when one hemisphere is pointed towards the Sun (i.e. in summer), it will experience 42 years of continuous light. In winter, the situation is reversed, with this same hemisphere experiencing 42 years of continuous darkness. A Year On Neptune:Given its distance from the Sun, Neptune has the longest orbital period of any planet in the Solar System. As such, a year on Neptune is the longest of any planet, lasting the equivalent of 164.8 years (or 60,182 Earth days). But since Neptune also takes comparatively little time to rotate once on its axis (16 hours, 6 minutes and 36 seconds), a single year lasts a staggering 89,666 Neptunian days. What’s more, with an axial tilt close to Earth and Mars’ (28.5 degrees), there is some seasonal variation on the planet. Essentially, a single season lasts more than 40 years. But like all gas/ice giants, this does not result in noticeable temperature variations. We have written many interesting articles about the Solar System here at Universe Today. Here’s How Long Is A Year on Earth?, How Long Is A Year On Mercury?, How Long Is A Year on Venus?, How Long Is A Year on Mars?, How Long Is A Year On Jupiter?, How Long Is A Year On Saturn?, How Long Is A Year On Pluto?, and Orbits Of The Planets. If you are looking for more information, try NASA’s Solar System exploration page, and here’s a link to NASA’s Solar System Simulator. Astronomy Cast has episodes on all the planets, including Episode 49: Mercury, Episode 50: Venus, Episode 62: Uranus. |
Which planet A or B orbits the Sun in the lesser amount of time
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