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1973 Red Planet Mars Map

2 Apr

1973 Red Planet Mars Map

Explore the Red Planet in striking detail with this educational map published in February 1973. This map was half of a two-part piece, appearing with a complete index map entitled “The Dusty Face of Mars” and two additional articles on Mars: “Journey to Mars” and “The Search for Life on Mars.” Featuring detailed illustrations of Mars from different angles, this map also contains an abundance of illustrations and information about the planet’s physical attributes including atmosphere, surface color, magnetic field, winds, and temperature.

NASA’s Chandra X-ray Observatory (Andromeda) (Image)

12 Dec




28 Jun


Each type of twilight depends on how far the sun is below the horizon after sunset of before sunrise.  During civil twilight, there’s enough light to find your way around with ease and Venus is visible; in nautical twilight the horizon is indistinct and the first stars come out.  True dark night begins at the end of astronomical twilight.  10 degrees equals one fist held out at arm’s length against the sky.  Credit TW Carlson

Understanding Astronomy • Astronomy Before Copernicus (Internet Repost)

16 Jan

Understanding Astronomy

Astronomy Before Copernicus

There are three good reasons to study the history of astronomy. First, history itself is fascinating. Second, you’ll understand the facts of astronomy better if you know a little about why astronomers came to believe such incredible things. And third, the history of once-controversial ideas can shed light on scientific controversies that are still alive today.

This chapter begins the story of the greatest scientific controversy of all time: the battle over earth’s place in the universe. Is the earth unique, occupying a special place at the center of the universe? Or is it just another planet, drifting through space like the rest of the heavenly bodies? Today, every school child is taught that the second view is correct. But only a few hundred years ago, this view was considered absurd and even blasphemous. Let’s try to understand why.

The Heavens and the Earth

To our senses, the distinction between the heavens and the earth is totally obvious. The heavens are full of luminous objects in eternal motion, while the earth is a dark mass of rock and water where nothing keeps moving for very long. Everyone can see that the earth doesn’t move, while the motions of water and wind seem to be caused by influences from above. Even humans and animals, which can put themselves into motion at will, eventually die and decay. Because heavenly motions never cease, and the heavenly lights never burn out, the heavens must be supernatural—populated by immortal deities.

A depiction of Ra, the ancient Egyptian sun god, in the boat that carried him across the heavens. (Wikimedia Commons.)

Long before the invention of writing, different cultures developed different stories about the heavenly deities. The sun was often the most important of the gods, while other gods were associated with the moon, planets, and stars. Our modern names for the planets, and for many of the constellations, are based on the particular mythologies that developed in and around ancient Greece.

Ancient Greek Astronomy

But eventually, as described in earlier chapters, the Greeks realized that the heavenly motions are far too regular and repetitive to be attributed to the whims of the gods. Instead, Greek thinkers began devising mechanical models to explain these motions: giant wheels or spheres, carrying the celestial bodies around us in grand circles. By the time of Plato and Aristotle, the consensus view among educated Greeks was the Two-Sphere Model of the universe (as Thomas Kuhn later called it): A spherical earth, fixed at the center of the universe, surrounded by an enormous celestial sphere, holding the stars and spinning around us once a day. The sun, moon, and planets were presumably somewhere in between, carried around in their circles by similar mechanisms.

This system became complicated, however, when the Greeks tried to account for the motions of the planets in a quantitative way. Especially problematic were the periodic reversals in the plantets’ motions, from forward to retrograde and back again. Even the sun and the moon seem to move at slightly different speeds at different times, complicating predictions of their rising and setting times—not to mention eclipses. Quite a few Greek thinkers were curious about these complexities. Also, astronomers who could make accurate predictions found that they could sell their services in the form of astrological horoscopes.

In the ancient Greek epicycle model of planetary motion, each planet is affixed to a small wheel (the “epicycle”) whose center is affixed to a large wheel (the “deferent”). The simultaneous motion of the epicycle and deferent causes the planet to follow a loop-de-loop path, shown in red.

Greek astronomers devised many mechanisms to account for the irregularities of planetary motion. Among these mechanisms, the most famous is the deferent-epicycle system, shown in the illustration at right. In this model, each planet is carried around us by a combination of two invisible wheels. The larger wheel, called the “deferent”, is centered on the earth (at least approximately) and accounts for the planet’s forward motion. But instead of being attached directly to the deferent, the planet is attached to the rim of a smaller wheel, the “epicycle”, whose center is attached to the deferent. The planet’s actual path, therefore, makes loop-de-loops around the deferent as each wheel turns at its own rate. From earth’s perspective, the planet moves in the “forward” direction most of the time, but reverses to undergo retrograde motion during the innermost part of each loop.

To better understand the deferent-epicycle system, I highly recommend that you spend some time with the Ptolemaic System Simulator, a web applet created by the Nebraska Astronomy Applet Project. Notice how you can adjust the epicycle’s size and turning rate to produce a wide variety of looping planetary motions. A handy menu adjusts the settings to model the motion of Venus, Mars, Jupiter, or Saturn.

The basic deferent-epicycle model can account for the forward and retrograde motion of the planets. But to match the observed planetary motions in quantitative detail, ancient Greek astronomers had to introduce further complications: off-setting the earth from the center of the deferent; varying the rate at which the deferent turns during its rotation; and so on. This effort culminated in the work of Ptolemy, an astronomer who lived in Alexandria during the second century A.D. His astronomical treatise, known today by its Arabic title Almagest, became the authoritative reference on the subject throughout the Arab world and Europe for the next 1400 years. Although the calculations were complex, astronomers could use the system of Ptolemy’s Almagest to predict planetary positions with reasonable accuracy.

[Insert exercises based on Ptolemaic simulation]


Let’s face it: Most people couldn’t care less about the technical details of planetary motion. Let the experts do their calculations however they like! But most of us do care about the Big Picture: the overall shape of the universe, the relationship of the earth to the heavens, and the reasons why these things are the way they are. The study of these grand matters is called cosmology.

This illustration from Peter Apian’s Cosmographia (first published in 1524) shows a typical medieval view of cosmology. At the center of the universe is the earth, surrounded by layers of water, air, and fire. Then come the spheres of the moon, sun, planets, and stars. Finally, surrounding the universe, is the Empyrean Heaven, dwelling place of God. (Wikimedia Commons.)

Magellanic Clouds and More • EarthSky.Org (Internet Repost)

27 Nov

Magellanic Cloud

The Milky Way, Large and Small Magellanic Clouds and bright star Canopus can be seen in this image taken at sunrise over East Java’s Mount Bromo. September is one of the best months to see four galaxies — Large Magellanic Cloud, Small Magellanic Cloud, Andromeda Galaxy and Milky Way Galaxy in the Southern Hemisphere — within one night in Bromo.

The brightest star in the Southern Hemisphere, Canopus, can also be found between our Milky Way galaxy (dense area of stars toward the left) and the Large Magellanic Cloud.

The dense area of stars toward the left is actually the Orion Arm of the Milky Way galaxy.

This is a composite image of the sky and the foreground taken at the same location but at different time and I blended the images taken at different dynamic ranges manually in post processing.

Time to look for Mira the Wonderful, a famous variable star

14 Aug




Mercury and Venus in Conjunction (Right Ascension) on June 20, 2013

16 Jun

A conjunction occurs when two astronomical objects have either the same right ascension or the same ecliptical longitude

Mercury and Venus in Conjunction (Right Ascension) on June 20, 2013

Right Ascension (i.e. along the celestial equator) and the other is measured along the ecliptic, which is inclined at 23½° to the Earth’s equatorial plane (this is due to the tilt of the Earth’s axis in space).

Celestial Equator and Ecliptic

sphaera recta

How right ascension got its name. Ancient astronomy was very concerned with the rise and set of celestial objects. The ascension was the point on the celestial equator (red) which rose or set at the same time as an object (green) on the celestial sphere. As seen from the equator, both were on a great circle from pole to pole (left, sphaera recta or right sphere). From almost anywhere else, they were not (center, sphaera obliqua or oblique sphere). At the poles, objects did not rise or set (right, sphaera parallela or parallel sphere). An object’s right ascension was its ascension on a right sphere.