Helios and the Celestial Sphere As we’ve previously established, the celestial sphere rotates around the Earth at a constant speed, completing one turn every twenty-three hours and fifty-six minutes. Given our modern understanding of astronomy though, we the Earth has also moved 1/365th of its path around the sun in the same time frame. Due to this movement through space, the Earth has to rotate just a bit more before the sun appears back in the same place where it started. To be precise, it has to rotate for an extra four minutes, thus our 24 hour day. Since we use a solar day, the celestial sphere rotates slightly faster than the Earth’s orbit around the Sun, so the stars appear to rise four minute earlier each day. Four minutes daily, over 365 days, equals 1,460 minutes. Divided by 60 minutes per hour, that results in 24 hours, so each year, the stars return to their starting lines. The pole star doesn’t move, and the circumpolar stars never set, although the appear in different places at different times of the year. All the other stars and constellations appear above or below the horizon, depending on the time of night and year. Orion, for example, is known as a winter constellation, in the northern hemisphere, because it is not visible in summer. This correlation between what stars are visible and the time of year, reveal a celestial calendar of the year, as does the rising and setting of the sun at different angles from due east and due west. “Heliacal” rising is a phrase used to determine what day of the year a given star becomes visible just before sunrise. It marks the first time a star1 becomes visible again, after setting with the sun in a previous sunset. Since this occurs on the same day, each year, at the same place, when viewed from the same location on Earth, it proved extremely useful for preindustrial cultures. One traditional use for the knowledge of the heliacal cycles of the stars, was the timing of the planting cycles. A more familiar reference, for many historians, will the Egyptian to the heliacal rising of the brightest star in the night sky, Sirius. At the time of the ancient imperial power of the Pharaohs, the heliacal rising of Sirius, in that part of the world, reliable occurred just before the annual floods of the Nile, providing ample warning. It was also considered the beginning of the Egyptian New Year, when the star was known as Sothis. Similarly, the rising and setting of the constellation Pleiades, was used as a reference for grain growing across the Mediterranean.2 The Planets The planets are significantly less useful than the stars for navigation. Despite the fact that the planets don’t move in the same way the stars do on the celestial sphere, and they don’t actually look like stars, they are commonly mistaken for stars. The simplest way to distinguish a planet is by their appearance. Planets, when viewed through a telescope, are visible as small discs, or crescents. When viewed with the naked eye, they look similar to stars in brightness, color, and shape, but because they are so much closer than the stars, the light quality is different. Starlight, being from so much greater a distance, comes from a pinprick surface, when it hits the upper atmosphere. As this light filters through the atmosphere, it is reflected, refracted, and bounced around, causing the light to “twinkle” when we view it.3 The light from the astronomically4 closer planets appears much broader, so though it still gets bounced around, it will appear to be a much more constant, steady light. Planets then, don’t appear to “twinkle.” The word “planet” is derived from the Greek “planetai,” meaning “wanderers.” While the planets, like the Earth, are held in dependably regular orbits around the sun, via gravity, our own orbit complicates things, due to a phenomenon known as “retrograde motion.” Those planets that sit further from the Sun than the Earth does orbit much more slowly than we do. While we revolve around the sun in 365 days, it takes Jupiter 4,333 days, or twelve Earth years, to make the sun revolution. This means it appears to move slowly, one direction in the sky, until we pass on the inside turn, after which, it appears to move briefly in the other direction. Due to this retrograde motion, the position of each planet in the night sky at any specific time, unfortunately, cannot be determined by natural means. Instead, the complexity of the apparent motion of any particular planet will require the use of published tables or software to calculate. Leave it to the astronomers, or the ship’s skipper, with room in his berth of those charts. Fortunately, despite all this, a couple of basic concepts are still useful. First is that, once located and identified, the movement in the sky of any given planet will be minimal over the course of a few nights. Thus, once you’ve positively identified it as a planet, Jupiter, for example, will maintain its position relative to the stars around it. Second, because all of the planets in our solar system revolve on a similar plane around the sun5, they will appear within a band of about thirty degrees from the celestial equator.6 The fact is, the night sky has proven a reliable navigational aid around the globe, for thousands of years, and probably for hundreds of thousands of years, since the rise of anatomically modern humans, some 200,000 years ago in prehistory. Long before the advent of the GPS, or even before the development of the magnetic compass, the stars allowed people to travel and explore distant realms, and still find their way home again. It is possible to learn to find north in a matter of minutes, but don’t let that convenience breed complacency. The stars continue to move, but the day’s light will mask even the most visible stars in the night sky. If you’ve managed to use Polaris to locate north, it behooves you to mark your directions, so you’ll still have a solid reference in the morning’s light. During the Texas cattle drives north to the Kansas railroads, in the post-Civil War period, one method commonly used was to use the north star as a guide in the evening, and align the tongue of the cook’s wagon to indicate that direction, in the morning. 1Or planet, as we’ll see. 2While the “precession of the equinoxes” is a thing, and very real, the fact is, the position of the stars on the celestial sphere does not change over the course of even a handful of human lifetimes. As we’ve seen, the pole star is actually about one degree from the celestial pole. In 10,000 years though, the pole star will be Deneb, while in 14,000 years, Vega will be the the pole star. For the Egyptians, of 3000BCE, the star Thuban, in the constellation Draco, was the closest to the celestial pole, and is thus what was likely used to align the pyramids. Unless you’re going to live at least a handful of millenia though, you can safely ignore the precession of equinoxes and the movement of the stars. Focus on the locations and trajectories of the stars, as they exist today, and you’ll be good for the rest of your life. 3Thus the classic nursery rhyme, “Twinkle, Twinkle, Little Star.” 4Yeah, I went there… 5A fact that no science teacher, in the entirety of my academic career, was ever able to explain to me, despite my inquiries beginning around the fourth grade. 6Of the planets though, only Mercury, Venus, Mars, Jupiter, and Saturn are readily visible to the naked eye. Neptune, Uranus, and Pluto—if we’re considering Pluto a planet this week—are not visible without magnification. Check out the rest of the ongoing series here: https://www.patreon.com/posts/primi...paign=postshare_creator&utm_content=join_link Continue reading...
