Sketches from a Celestial Sea - One Drop

That is why the post has a simple and clear diagram accompanying it. Please have a look and you should find that it clarifies both the described methods for you. I read all these methods here, but the most obvious one that is missing is simply drawing a line between Beta Centauri Hadar and Achernar, and dividing it by two. Yet another, when Achernar is too close to the southern horizon, is to use Epsilon Carinae at the bottom of the False Cross and Alpha Pavonis named Peacock or humorously Pavlova , whose line is similarly divided in half. Actually, Hadar , Achernar, Epsilon Car and Peacock form a celestial cross, whose centre is always the south celestial pole.

No matter what time of night or part of the year, at least two of these stars can be seen. An isosceles triangle, with the place of the pole between any of the two selected stars, is always at about 90 degrees. All of these work from latitudes of Sydney, degrees, and south of it, finds the south celestial pole. Are you being modest Mr. Lomb, or have you been using your time with the equipment for more nefarious activities?

The above post presents two well-known methods of finding south using the Southern Cross. It is suitable for use in highly light-polluted areas like Sydney and by people who do not have a detailed knowledge of the sky. Daniel Fischer kindly shared another method that may be best suited to darker conditions and for use by people with some familiarity with the sky.

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Now readers have a choice of methods, which is great. A method I personally found easily understandable, in fact. Many countries often have unique different methods and perspectives on finding south. Also from Namibia, which is much closer to the equator, using Crux to find south celestial pole year round is not very good because it is not circumpolar. I can understand why using stars closer to the south celestial pole would be desirable because they would be truly circumpolar. In a city like Sydney, we are lucky to see 2nd or 3rd magnitude stars, but the stars by the method you elude are quite invisible.

Extensive light pollution in the central city of Sydney is just atrocious, and is now especially poor where Sydney Observatory is located, you are often lucky to see a few dozen stars. Most observers living is Sydney have to travelon average fifty or sixty kilometres to even get to dark skies to see 6th magnitude, and over one hundred kilometres to seeing the lower 6.

It is certainly not like the killer dark skies where you are so luckily observing! There are so many other urban things to use, that doing so would be plainly inane. If some method is to be used it must be memorable to the novice, easy to remember, and universally accepted. Because of the difference in the gravitational pull from the Sun on opposite sides of the Earth is much smaller, the Sun's tendency to make the Earth bulge is much less. Still, the contribution to tides from the Sun is noticeable. When the Sun, Moon and Earth are aligned, the Sun adds to the Moon's tidal pull, making the tides greater.

This is called the spring tide. The spring tide has no connection with the season of spring. When the Moon is at a right angle with the Sun, the Sun's tidal pull interferes with the Moon's, making the tides weaker. This configuration is called the neap tide.

Why do we have seasons? A little thought will suggest that it can't have much to do with the Earth's distance from the sun, as that would affect the Southern and Northern Hemispheres at the same time. In fact, the Earth is slightly nearer to the sun around December than at other times of the year. Why, then are there seasons? Every year the Earth completes one orbit of the Sun.

We see this observe the effect as the change of the seasons and the movement of the constellations. Over the course of a year, the Sun moves through a great circle on the celestial sphere, tracing out the same path year after year. This path is called the ecliptic. The ecliptic is not only the path of the Sun in the sky, it also marks the plane of the Earth's orbit of the Sun. The planets orbit the Sun in different planes but near to the ecliptic. Globes are typically built with an inclined rotation axis.

In the Northern Hemisphere, the Sun will pass directly over head only between June 20 and 22th along the Tropic of Cancer. That day is called the day of the summer maximum or Solstice in the Northern Hemisphere. In the Southern Hemisphere, the Sun will pass directly over head only once between December 20 and 23rd along the Tropic of Capricorn.

Anywhere between the Tropic of Cancer and the Tropic of Capricorn, the Sun will pass directly overhead at least twice during the year, but the Sun will never pass overhead for people living outside the tropics. Within the Tropics, over the course of a year, the Sun's position in the sky changes, beginning in the southern sky about December 21, moving to the northern sky in mid-year, and ending the year back in the southern sky.

When the sun lies at one of the intersections, it is directly overhead somewhere on the equator. This occurs at the equinox , and the points on the sky where the equinox intersects the equator are also called equinoxes. Once every year, the Sun passes through the equator going north.

This happens in late March — the "vernal" or "spring" equinox. The "autumnal" equinox occurs when the Sun passes through the equator in late September. On the equinox days, the day and night are equally long. This is the origin of the name equinox, which is from Latin for "equal night. It doesn't rise to directly overhead, though, except for observers on the Equator. The equinoxes are the only days of the year that have twelve hours of daylight and twelve hours of dark. After the vernal equinox, moving into Northern summertime, the Sun begins rising in the northeast and sets in the northwest.

Days in the Northern Hemisphere become longer, while days in the Southern Hemisphere become shorter. The points at which the Sun is at its greatest distance from the equator are called the solstices. The solstices mark the longest and shortest day of the year. The longest day of the year is the summer solstice and the shortest day is the winter solstice.

In the Northern Hemisphere, the summer solstice occurs when the Sun is farthest north, while the winter solstice occurs at the Sun's southernmost point. In the Southern Hemisphere, the solstices are reversed. Viewed from space, we see that the Earth's tilt changes the exposure of different parts of the Earth to the Sun. Observers in the Northern Hemisphere will see the Sun at its lowest position in the southern sky, about December They see it this way because the Southern Hemisphere is tilted towards the Sun and the Northern Hemisphere is tilted away.

About June 22, the situation is reversed, with the Northern Hemisphere pointed toward the Sun, and the Sun will be in its extreme high point in the sky at solar noon. For an observer in the Southern Hemisphere, the Sun will appear at its lowest point in the sky in the north, about June 22, while the Sun will appear at its high point in the sky about December One effect of this phenomenon is that during the months of Northern Hemisphere summer, the North Pole will be able to receive sunlight twenty-four hours a day. The Sun will remain visible through much of the autumn, passing below the horizon at the autumnal equinox.

As winter sets in at the North Pole, the Sun will not be seen for six months, while that portion of the Earth is tilted away from the Sun. As one moves toward the Earth's equator from either pole, this effect becomes less severe. The nearer one is to the equator, the less difference there will be between the number of hours of illumination and night hours. At the equator, there's practically no difference between the length of day all through the year.

Clearly, the annual motion of the Earth around the Sun is the cause of Earth's seasons. What effect gives rise to this seasonal change is less obvious. At first glance, one might think that winter occurs when the Earth is farther from the Sun. If we realize the Northern and Southern Hemispheres have winter at different times of year, we see that this can't be right.

Also, the Earth's orbit is very nearly circular. The change in the Earth's orbital distance is much too small to have a noticeable effect on Earth's climate. Certainly the length of time each day during which sunlight falls on a particular location has a great deal to do with the seasonal changes in temperature. However, another effect less obvious, but more influential is the angle at which the sunlight hits a region.

At the equator, there is little difference throughout the year as the Sun varies by The length of the ray's path on Dec 21 at solar noon is increased by a factor of only 1.

General Astronomy/Observational Astronomy

The more direct radiation gives the maximum amount of heat and energy to the earth where it falls, and therefore these areas will receive the most warmth. Away from the equator, however, the Earth's tilt means that sunlight is not received so directly and a greater amount of the Sun's energy is blocked by the longer path it takes through the atmosphere. At 50 degrees north latitude, the path the Sun's rays travel through the atmosphere on Dec 21, at solar noon, will be increased by a factor of 3.

In general, the rays will come at an angle that depends on the time of day, the latitude of the region from the equator, and the position of Earth in its orbit. The constellations in the ecliptic, the zodiac , have a long history in the tradition of astrology. In most newspapers, you can read a completely unscientific prediction of your future or some personal advice, specific to your birthday. Each entry is associated with one of the constellations of the zodiac and a range of birth dates.

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In the tradition of astrology, the constellation the Sun occupied on your birthday, your "sign," reveals information about your personality and your future. Interestingly, the dates given for each constellation in the newspaper don't match the Sun's position in the sky for those dates.

There is a mismatch between the date in the newspaper and the real position of the Sun of a little more than a month. The mismatch appears because the dates corresponding to each sign were set thousands of years ago. Over the course of thousands of years, the Earth "wobbles" on its axis, causing the calendar and the positions of the stars in the sky to shift.

This wobble is caused by the pull on the equator by the sun and moon, and is called precession. It affects the positions of all the constellations with respect to the equinoxes and the pole. The precession of the Earth is like the movement of a top. If you spin a top with the axis tilted, the axis will slowly rotate as the top spins. Since precession changes the direction in which Earth's pole points, it also changes which star is the North Star, if any. Earlier, we quoted Shakespeare, who referenced Polaris in Julius Caesar , describing it as the northern star.

Strictly, this would be incorrect. Polaris was not "fixed" in the sky in Julius Caesar's time because Earth's axis was pointed differently, toward the Big Dipper. Precession is a slow drift, and a difficult motion to detect. The motion of the stars from precession only becomes noticeable to the unaided eye after many, many years of careful observation, although it becomes very quickly noticeable through a telescope.

The Greek astronomer Hipparchus was the first to measure the precession by comparing his own observations to observations collected a century and a half before. Precession changes the position of the Earth in its orbit for the solstices and the equinoxes. As the Earth's axis turns over, the moment when it points most closely towards the Sun changes, and so the seasons change.

If a calendar didn't account for this, the seasons would drift as the axis precessed. Eventually, the Northern Hemisphere would be cold in July and warm in January, and the Southern Hemisphere would have warm July weather and cold January weather. The calendar takes the extra motion of precession into account by using the tropical year as its basis. The year as we usually define it is a sidereal year , the time it takes for the Earth to make one orbit around the Sun.

In one year, as we usually measure it, the Earth really completes a little more than a full orbit around the Sun. During a sidereal year, the Sun moves fully around the sky and back into the same position with respect to the stars. In a tropical year, the Sun goes from the Vernal Equinox, around the sky, and back to the Vernal Equinox again.

During this time, the equinox has shifted slightly in its position, so that a tropical year is a bit shorter than a sidereal year. It's easy to identify the progression of the calendar if you take careful notice of the sky. Next time you see sunrise or sunset, take notice of whether the Sun is setting due west or just north or south of west. Many ancient cultures watched the motion of the Sun carefully and over long periods of time.

Using simple techniques and tools, they were able to measure periods like the length of a year very accurately. Ancient people who took notice of celestial motion would have found that the summer solstice occurred every days. They would also notice that the solstice was delayed an extra day every four years. This is the reason for the leap year in the modern calendar. By taking some simple observations over a period of a few years, it is possible to measure the length of a year to surprising accuracy using this technique.

Solar calendars have been used throughout history. The ancient Babylonians thought the year had only days, and made their calendar accordingly. The Hebrew calendar is lunisolar.

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Our modern calendar is handed down to us from the Ancient Roman civilization. The calendar took its first mature form as the Julian calendar , almost exactly the same as the one used today. It had days in a year, with a day leap year every four years. In the Julian calendar, years divisible by 4 — such as , , and — are leap years. Although the drift of the Julian calendar is slow, the error in the calendar had accumulated enough by the sixteenth century that the Catholic Church became concerned about the drift's effect on the date of the celebration of Easter. The Italian chronologer Aloisius Lilius invented modifications to the Julian calendar to correct the difference.

Pope Gregory XI instituted the new calendar, now named the Gregorian calendar , in the year The Gregorian calendar was identical to the Julian calendar except that the leap year was skipped on years not divisible by In the years , , and , there would be a leap year in the Gregorian calendar, but not , , or This produced a year of average length The Gregorian calendar accumulates only 3 days of error over 10, years. Draw a quick sketch that shows the relative positions of the sun and the earth on those dates.

Suppose you are an astronomer in America. You observe an exciting event say, a supernova in the sky and would like to tell your colleagues in Europe about it. Suppose the supernova appeared at your zenith. You can't tell astronomers in Europe to look at their zenith because their zenith points in a different direction.

You might tell them which constellation to look in. This might not work, though, because it might be too hard to find the supernova by searching an entire constellation. The best solution would be to give them an exact position by using a coordinate system. On Earth, you can specify a location using latitude and longitude. This system works by measuring the angles separating the location from two great circles on Earth namely, the equator and the prime meridian.

Coordinate systems in the sky work in the same way. The equatorial coordinate system is the most commonly used. The equatorial system defines two coordinates: The declination is the angle of an object north or south of the celestial equator. Declination on the celestial sphere corresponds to latitude on the Earth. The right ascension of an object is defined by the position of a point on the celestial sphere called the vernal equinox. The further an object is east of the vernal equinox, the greater its right ascension.

A coordinate system is a system designed to establish positions with respect to given reference points. The coordinate system consists of one or more reference points, the styles of measurement linear measurement or angular measurement from those reference points, and the directions or axes in which those measurements will be taken. In astronomy, various coordinate systems are used to precisely define the locations of astronomical objects.

Latitude and longitude are used to locate a certain position on the Earth's surface. The lines of latitude horizontal and the lines of longitude vertical make up an invisible grid over the earth. Lines of latitude are called parallels. Lines of longitude aren't completely straight they run from the exact point of the north pole to the exact point of the south pole so they are called meridians. It was finally agreed that the observatory in Greenwich, U. Latitude and longitude are measured in degrees.

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One degree is about 69 miles. There are 60 minutes ' in a degree and 60 seconds " in a minute. There are a few main lines of latitude: The Antarctic Circle is The Arctic Circle is an exact mirror in the north. The Tropic of Cancer separates the tropics from the temperate zone. It is mirrored in the south by the Tropic of Capricorn. One of the simplest ways of placing a star on the night sky is the coordinate system based on altitude and azimuth, thus called the Alt-Az or horizontal coordinate system.

The reference circles for this system are the horizon and the celestial meridian, both of which may be most easily graphed for a given location using the celestial sphere. In simplest terms, the altitude is the angle made from the position of the celestial object e. The azimuth is the angle from the northernmost point of the horizon which is also its intersection with the celestial meridian to the point on the horizon nearest the celestial object. Usually azimuth is measured eastwards from due north. An object's altitude and azimuth change as the earth rotates.

The equatorial coordinate system is another system that uses two angles to place an object on the sky: The ecliptic coordinate system is based on the ecliptic plane, i. The great circle at which this plane intersects the celestial sphere is the ecliptic, and one of the coordinates used in the ecliptic coordinate system, the ecliptic latitude, describes how far an object is to ecliptic north or to ecliptic south of this circle. On this circle lies the point of the vernal equinox also called the first point of Aries ; ecliptic longitude is measured as the angle of an object relative to this point to ecliptic east.

Since we are inside the Milky Way, we don't see the galaxy's spiral arms, central bulge and so forth directly as we do for other galaxies. Instead, the Milky Way completely encircles us. We see the Milky Way as a band of faint starlight forming a ring around us on the celestial sphere.

The disk of the galaxy forms this ring, and the bulge forms a bright patch in the ring. You can easily see the Milky Way's faint band from a dark, rural location. Our galaxy defines another useful coordinate system — the galactic coordinate system. This system works just like the others we've discussed. It also uses two coordinates to specify the position of an object on the celestial sphere. The galactic coordinate system first defines a galactic latitude, the angle an object makes with the galactic equator. The galactic equator has been selected to run through the center of the Milky Way's band.

The second coordinate is galactic longitude, which is the angular separation of the object from the galaxy's "prime meridian," the great circle that passes through the Galactic center and the galactic poles. The galactic coordinate system is useful for describing an object's position with respect to the galaxy's center. For example, if an object has high galactic latitude, you might expect it to be less obstructed by interstellar dust.

One can use the principles of spherical trigonometry as applied to triangles on the celestial sphere to derive formulas for transforming coordinates in one system to those in another. These formulas generally rely on the spherical law of cosines, known also as the cosine rule for sides. By substituting various angles on the celestial sphere for the angles in the law of cosines and by thereafter applying basic trigonometric identities, most of the formulas necessary for coordinate transformations can be found. The law of cosines is stated thus:.

The Celestial Sphere

Using the same symbols and formulas, one can also derive formulas to transform from equatorial to horizontal coordinates:. Transformation from equatorial to ecliptic coordinate systems can similarly be accomplished using the following formulae:. Again, using the same formulas and symbols, new formulas for transforming ecliptic to equatorial coordinate systems can be found:. From Wikibooks, open books for an open world. Retrieved from " https: Views Read Latest draft Edit View history.

Policies and guidelines Contact us. In other languages Add links. Gods, too, proliferate, are subject to evolution, diversification. Errors in the transmission codes myth, ritual creep in; crises come, circumstances change. There are gods, therefore, not only of the sky but of the earth, of the forest, of the sea. The Greeks had their executive Olympians each with his own local face and their deep-sky progenitors the Titans, and before them Gaia and Uranus, primordial deities , but also ten thousand local gods, asterised heroes; no peasant, shepherd or slave did not tend a grubby local shrine to some local deity, more loved and feared than any state-sanctioned Apollonian worship; and the wilderness beyond was even more compacted with wild things, tree spirits, river gods, centaurs, Pan.

The Egyptians, not uniquely but with unique precision, proliferated their gods also in time. The celestial clock was divided not only into the twelve hours of the zodiac but also into minutes and seconds; each of the zodiacal signs is sub-divided into three, and these thirty-six decans, as they are known, preside in turn over each moment of the day and night in turn.

The ontology of gods also shifts. There is an episode of Star Trek: The Original Series , in which the Olympian gods turn out to have been a gang of alien chancers enjoying for a spell the credulity and adulation of the Greeks. The Q character in The Next Generation and Voyager is immortal, omnipotent, but not a god, merely a special alien.

The resident gods of Bajor in Star Trek: Deep Space Nine , are variously referred to as prophets and as wormhole aliens. There are henotheistic religions—Hinduism, for example—where the endless family of gods are considered to be just aspects of the one true god. There are Christianities where God is in nature or beyond it, or where God is located somewhere in the cosmic background radiation, back at the big bang. In Norbiton we pride ourselves on being a city of free men and women, and that freedom includes—demands—freedom from delusion; however, perhaps after all we do have our genii loci , our popular cults and shrines; perhaps we are no different from the grubby shepherds leaving small votive objects by sacred rocks, trying in vain to draw down the beneficence of an impotent invisible god, the spiritus mundi of success; perhaps, we will be asterised heroes to future inhabitants of Norbiton: Monkey mummy, Cairo Egyptian Museum — Photo: Chthonic deities are gods of the earth rather than gods of the middle air or of the celestial sphere.

They are older, wiser, sleepier. They are not a success. The Olympians have recourse to the chthonic deities when they are in trouble. Otherwise they forget about them.



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She is just a woman, albeit a woman who is hard to get hold of; and a resolutely pleasant woman. Botticelli, it has been argued , painted his Primavera ? What he was making, knowingly or not, was a talisman. The painting is read, right to left, as a progression from earthly or sensual love to spiritual enlightenment what else is the breath of Zephyrus if not the exhalation of the spiritus mundi?

Venus, in the centre, is starting to dance or making a gesture of welcome. The painting, according to this argument, is a not a representation but a vehicle of spiritus , a transubstantiated object. Whether this understanding of the painting is true or otherwise is in a sense irrelevant: Notable about it, apart from a sort of genial barminess of conception, is its lack of spatial depth. Venus is slightly set back from the hieroglyphic pageant at which she is present, and there is a narrow foreground of sympathetic?

But this is a two-dimensional map of heaven, more trecento than quattrocento. Or rather, like Byzantine icons, the space which the figures are proposed to occupy is in front of the picture plane — i. In retreating from a fully articulated perspectival construction otherwise assuredly present in his work Botticelli is reinstating the picture plane as something made, something you share a space with; not something you look through.

It makes an appearance in the world of objects with weight. It is not a representation, does not seek to render itself invisible; it is a mass of paint, canvas, stretchers, frame. Its existence, to coin a cheap phrase, precedes its essence. I said that the object was axiomatically talismanic, and I need to justify that; and I would do so, not by insisting on the magical properties of all weighty objects, but by emptying the word talismanic of its magical or supernatural content. An object that is in some way or other hyper-present, with which you share space, has necessarily a gravitational pull, is part, moreover, of a gravitational system of like-minded objects; and, being objects, not only do you enter into a sensory relationship with them—you can see them, touch them, knock them together, move them about—but you can interfere in their patterns, impose upon them, operate upon them, destroy them if need be.