Armillary Sphere

Armillary sphere with astrononomical clock Credit: Chris Bainbridge


Despite the fact that the term “armillary sphere” sounds like a high-tech weapon or something from a science fiction movie, it is neither. An armillary sphere is an old tool that is supposed to represent the heavens. They were models of what scientists thought the heavens looked like and how they were suppose to have moved. The armillary sphere is also known as the spherical astrolabe, the armilla, or the armil. The armillary sphere is related to the astrolabe, which was a navigation tool used for determining the position of the Sun and stars and used by sailors for navigating.

The armillary sphere was invented hundreds of years ago. The identity of who created the sphere has been debated. Some credit its invention to a Greek named Eratosthenos. Others have said that the Chinese or other Greek scholars invented it. Regardless of its inventor, the armillary sphere is one of the oldest astronomical instruments in the world. In addition to its being used in the Greek world, the armillary sphere was also used throughout Asia and the Islamic Empire.

These devices were used as teaching tools and models. The models were used to show the difference between the Ptolemaic and Copernican theories of the Solar System. In the Copernican theory, the Sun is the center of our Solar System, while the Earth is the center of the Solar System according to the Ptolemaic theory. When armillary spheres were first invented, the Ptolemaic theory was still the accepted view. It was soon after armillary spheres were invented that Copernicus set forth his theory of the Sun as the center of the Solar System, although it was not widely accepted until centuries later.

The armillary sphere looks like a sphere circled by a ring and set upon a base. Armillary spheres were made with different numbers of circles arranged at various angles. Spheres with both four and nine circles have been known to exist – as well as ones with different numbers. These rings would then be adjusted in order to trace the path of the stars.

The armillary sphere also turns up in the Portugal flag, originally as a symbol for the country’s colony Brazil. The armillary sphere was widely used for navigating at sea, and exploration was heavily promoted by the Portugese royalty. In the early 1800’s, the sign was removed from the national flag when Brazil gained its independence. However, it was replaced in 1911 after Portugal became a Republic. You can still purchase armillary spheres today, although some of them are extremely expensive, especially if they are antiques.

Universe Today has articles on Solar System projects  and parallax.

If you are looking for further information, check out how to build an armillary sphere and astrolabes.

Astronomy Cast has an episode on telescopes.

Source: Wikipedia

Hertzsprung-Russell diagram

The Hertzsprung-Russell Diagram.

Stars can be big or small, hot or cool, young or old. In order to properly organize all of the stars out there, astronomers have developed an organizational system called the Hertzsprung-Russell Diagram. This diagram is a scatter chart of stars that shows their absolute magnitude (or luminosity) versus their various spectral types and temperatures. The Hertzsprung-Russell diagram was developed by astronomers Ejnar Hertzsprung and Henry Norris Russell back in 1910.

The first Hertzsprung-Russell diagram showed the spectral type of stars on the horizontal axis and then the absolute magnitude on the vertical axis. Another version of the diagram plots the effective surface temperature of the star on one axis and the luminosity of the star on the other.

By using this diagram, astronomers are able to trace out the life cycle of stars, from young hot protostars, through the main sequence phase and into the dying red giant phases. It also shows how temperature and color relate to the stars at various stages in their lives.

If you look at an image of a Hertzsprung-Russell diagram, you can see there’s a diagonal line from the upper left to the lower right. Almost all stars fall along this line, and it’s known as the main sequence. In general, as luminosity goes down, temperature goes down as well. But there’s a branch that goes off horizontally at the 100 solar luminosity mark. These are the red giant stars nearing the end of their lives. They can be bright and cool, because they’re so large. But this stage usually only lasts a few million years.

Astronomers can also use the Hertzsprung-Russell diagram to estimate how far away stellar clusters are from Earth. By mapping out all the stars in the cluster and grouping them together and comparing them to groups of stars with known distances.

We have written many articles for Universe Today about the star life cycle. Here’s an article about the cluster M13, and how astronomers use the Hertzsprung-Russell diagram to study it.

Here are some good resources on the Internet for Hertzsprung-Russell diagram. Here’s a very simple version of the diagram from the University of Oregon, and here’s more information.

We have recorded an episode of Astronomy Cast about kinds of stars. Listen to it here, Episode 75 – Stellar Populations.


LookUP to Find Astronomical Objects

Sky map of Epsilon Aurigae

Have you heard about LookUP? Stuart Lowe from the Jodrell Bank Centre for Astrophysics created this web tool to provide quick access to information about the the position and other details of specific astronomical objects. Instead of having to go search through an astronomical database, all you have to do is type in the name of the object (this doesn’t apply for spacecraft) and LookUP contacts the relevant astronomical databases for you and provides info such as right ascension and declination. There’s also mobile version, an application for iPhones, and a widget for your desktop. The newest tool will thrill all the astronomy Twitterers out there. Rob Simpson from Orbiting Frog fame created a Twitter account for LookUP. All you do is send a tweet to it with the name of your object, and it will send you the info and a link with for further information. For example, I wanted to know where Asteroid Apophis was, and LookUp Tweeted back: Apophis is at RA 10:35:13.594 dec 07:37:40.210 More info (that is valid for the time I sent the Tweet.) Check it out; it’s all very quick and easy and wonderful for all you stargazers out there.


Binoculars for Astronomy

Astronomy is best when you get outside and look into the skies with your own eyes. And the best way to get started is with a set of binoculars for astronomy. They’re light, durable, easy to use, and allow you to see objects in the night sky that you just couldn’t see with your own eyes. There are so many kinds of binoculars out there, so we’ve put together this comprehensive guide to help you out.

Everyone should own a pair of binoculars. Whether you’re interested in practicing serious binocular astronomy or just want a casual cosmic close-up, these portable “twin telescopes” are both convenient and affordable. Learning more about how binoculars work and what type of binoculars work best for astronomy applications will make you much happier with your selection. The best thing to do is start by learning some binocular “basics”.

What are binoculars and how do they work?
Binoculars are both technical and simple at the same time. They consist of an objective lens (the large lens at the far end of the binocular), the ocular lens (the eyepiece) and a prism (a light reflecting, triangular sectioned block of glass with polished edges).

The prism folds the light path and allows the body to be far shorter than a telescope. It also flips the image around so it doesn’t look upside-down. The traditional Z-shaped porro prism design is well suited to astronomy and consists of two joined right-angled prisms which reflects the light path 3 times. The sleeker, straight barrelled roof prism models are more compact and far more technical. The light path is longer, folding 4 times and requires stringent manufacturing quality to equal the performance. These models are better suited to terrestrial subjects, and are strongly not recommended for astronomy use.

If you’re using binoculars for astronomy, go with a porro prism design.

Choosing the Lens Size
Every pair of binoculars will have a pair of numbers associated with it: the magnifying power times (X) the objective lens size. For example, a popular ratio is 7X35. For astronomical applications, these two numbers play an important role in determining the exit pupil – the amount of light the human eye can accept (5-7mm depending on age from older to younger). By dividing the objective lens (or aperture) size by the magnifying power you can determine a pair of binoculars exit pupil.

Like a telescope, the larger the aperture, the more light gathering power – increasing proportionately in bulk and weight. Stereoscopic views of the night sky through big binoculars is an incredible, dimensional experience and one quite worthy of a mount and tripod! As you journey through the binocular department, go armed with the knowledge of how to choose your binoculars lens size.

Why does the binocular lens size matter? Because binoculars truly are a twin set of refracting telescopes, the size of the objective (or primary) lens is referred to as the aperture. Just as with a telescope, the aperture is the light gathering source and this plays a key role in the applications binoculars are suited for. Theoretically, more aperture means brighter and better resolved images – yet the size and bulk increases proportionately. To be happiest with your choice, you must ask yourself what you’ll be viewing most often with your new binoculars. Let’s take a look at some general uses for astronomy binoculars by their aperture.

Different Sizes of Binoculars
Binoculars with a lens size of less that 30mm, such as 5X25 or 5X30, are small and very portable. The compact models can fit easily into a pocket or backpack and are very convenient for a quick look at well-lit situations. In this size range, low magnifications are necessary to keep the image bright.

Compact models are also great binoculars for very small children. If you’re interested in choosing binoculars for a child, any of these models are very acceptable – just keep in mind a few considerations. Children are naturally curious, so limiting them to only small binoculars may take away some of the joy of learning. After all, imagine the thrill of watching a raccoon in its natural habitat at sundown… Or following a comet! Choose binoculars for a child by the size they can handle, whether the model will fold correctly to fit their interpupilary size, and durability. Older children are quite capable of using adult-sized models and are naturals with tripod and monopod arrangements. For less than the price of most toys, you can put a set of quality optics into their hands and open the door to learning. Children as young as 3 or 4 years old can handle 5X30 models easily and enjoy wildlife and stargazing both!

Binocular aperture of up to 40mm is a great mid-range size that can be used by almost everyone for multiple applications. In this range, higher magnification becomes a little more practical. For those who enjoy stargazing, this is an entry level aperture that is very acceptable to study the Moon and brighter deep sky objects and they make wonderful binoculars for older children.

Binoculars up to 50-60mm in lens size are also considered mid-range, but far heavier. Again, increasing the objective lens size means brighter images in low light situations – but these models are a bit more bulky. They are very well suited to astronomy, but the larger models may require a support (tripod, monopod, car window mount) for extended viewing. Capable of much higher magnification, these larger binocular models will seriously help to pick up distant, dimmer subjects such as views of distant nebulae, galaxies and star clusters. The 50mm size is fantastic for older children who are ready for more expensive optics, but there are drawbacks.

The 50-60mm binoculars are pushing the maximum amount of weight that can be held comfortably by the user without assistance, but don’t rule them out. Available in a wide range of magnifications, these models are for serious study and will give crisp, bright images. Delicate star clusters, bright galaxies, the Moon and planets are easily distinguishable in this aperture size. These models make for great “leave in the car” telescopes so you always have optics at hand. For teens who are interested in astronomy, binoculars make an incredible “First Telescope”. Considering a model in this size will allow for most types of astronomical viewing and with care will last through a lifetime of use.

Binoculars any larger than 50-60mm are some serious aperture. These are the perfect size allowing for bright images at high magnification. For astronomy applications, binoculars with equations like 15X70 or 20X80 are definitely going to open a whole new vista to your observing nights. The wide field of view allows for a panoramic look at the heavens, including extended comet tails, large open clusters such as Collinder Objects, starry fields around galaxies, nebulae and more… If you have never experienced binocular astronomy, you’ll be thrilled at how easy objects are to locate and the speed and comfort at which you can observe. A whole new experience is waiting for you!

Binocular Magnification
When choosing binoculars for astronomy, just keep in mind that all binoculars are expressed in two equations – the magnifying power X the objective lens size. So far we have only looked at the objective lens size. Like a telescope, the larger the aperture, the more light gathering power – increasing proportionately in bulk and weight. Stereoscopic views of the night sky through big binoculars is an incredible, dimensional experience, but for astronomical applications we need these two numbers to play an important role in determining the exit pupil – the amount of light the human eye can accept. By dividing the objective lens (or aperture) size by the magnifying power you can determine a pair of binoculars exit pupil. Let’s take a look at why that’s important.

How do binoculars magnify? What’s the best magnification to use? What magnifying power do I choose for astronomy? Where do I learn about what magnifying power is best in binoculars? Because binoculars are a set of twin refracting telescopes meant to be used by both eyes simultaneously, we need to understand how our eyes function. All human eyes are unique, so we need to take a few things into consideration when looking at the astronomy binocular magnification equation.

By dividing the objective lens (or aperture) size by the magnifying power you can determine a pair of binoculars exit pupil and match it to your eyes. During the daylight, the human eye has about 2mm of exit pupil – which makes high magnification practical. In low light or stargazing, the exit pupil needs to be more around 5 to be usable.

While it would be tempting to use as much magnification as possible, all binoculars (and the human eye) have practical limits. You must consider eye relief – the amount of distance your eye must be away from the secondary lens to achieve focus. Many high “powered” binoculars do not have enough outward travel for eye glass wearers to come to focus without your glasses. Anything less than 9mm eye relief will make for some very uncomfortable viewing. If you wear eyeglasses to correct astigmatism, you may wish to leave your glasses on while using binoculars, so look for models which carry about 15mm eye relief.

Now, let’s talk about what you see! If you look through binoculars of two widely different magnifying powers at the same object, you’ll see you have the choice of a small, bright, crisp image or a big, blurry, dimmer image – but why? Binoculars can only gather a fixed amount of light determined by their aperture (lens size). When using high magnification, you’re only spreading the same light over a larger area and even the best binoculars can only deliver a certain amount of detail. Being able to steady the view also plays a critical role. At maximum magnification, any movement will be exaggerated in the viewing field. For example, seeing craters on the Moon is a tremendous experience – if only you could hold the view still long enough to identify which one it is! Magnification also decreases the amount of light that reaches the eye. For these reasons, we must consider the next step – choosing the binocular magnification – carefully.

Binoculars with 7X magnifying power or less, such as 7X35, not only delivers long eye relief, but also allows for variable eye relief that is customizable to the user’s own eyes and eyeglasses. Better models have a central focus mechanism with a right eye diopter control to correct for normal right/left eye vision imbalance. This magnification range is great for most astronomy applications. Low power means less “shake” is noticed. Binoculars with 8X or 9X magnification also offer long eye relief, and allows comfort for eyeglass wearers as well as those with uncorrected vision. With just a bit more magnification, they compliment astronomy. Binoculars 10 x 50 magnifying power are a category of their own. They are at the edge of multipurpose eye relief and magnifying power at this level is excellent across all subject matter. However, larger aperture is recommended for locating faint astronomy subjects.

Binoculars with 12-15X magnifying power offer almost telescopic views. In astronomy applications, aperture with high magnification is a must to deliver bright images. Some models are extremely well suited to binocular astronomy with a generous exit pupil and aperture combined. Binoculars with 16X magnification and higher are on the outside edge of high magnification at hand-held capabilities. They are truly designed exclusively as mounted astronomical binoculars. Most have excellent eye relief, but when combined with aperture size, a tripod or monopod is suggested for steady viewing. If you’re interested in varying the power, you might want to consider zoom binoculars. These allow for a variety of applications that aren’t dependent solely on a single feature. Models can range anywhere from as low as 5X magnification up to 30X, but always bear in mind the higher the magnification – the dimmer the image. Large aperture would make for great astronomy applications when a quick, more magnified view is desired without being chained to a tripod.

Other Binocular Features
The next thing to do is take a good look at the binoculars you are about to purchase. Check out the lenses in the light. Do you see blue, green, or red? Almost binoculars have anti-reflection coatings on their air to glass surfaces, but not all are created equal. Coatings on binocular lenses were meant to assist light transmission of the object you’re focusing on and cancelling ambient light. Simply “coated” in the description means they probably only have this special assistance on the first and last lens elements – the ones you’re looking at. The same can also be said of the term “multi-coated”, it’s probably just the exterior lens surface, but at least there’s more than one layer! “Fully coated” means all the air-to-glass surfaces are coated, which is better… and “fully multi-coated” is best. Keeping stray light from bouncing around and spoiling the light you want to see is very important, but beware ruby coated lenses… These were meant for bright daylight applications and will rob astronomical binoculars of the light they seek.

Last, but not least, is a scary word – collimation. Don’t be afraid of it. It only means the the optics and the mechanics are properly aligned. Most cheap binoculars suffer from poor collimation, but that doesn’t mean you can’t find an inexpensive pair of binoculars that are well collimated. How can you tell? Take a look through them with both eyes. If you can’t focus at long distance, short distance and a distance in-between, there is something wrong. If you can’t close either eye and come to focus with the other, there’s something wrong. Using poorly collimated binoculars for any length of time causes eye strain you won’t soon forget.

Price range for Astronomy Binoculars
So, how much? What does a good pair of binoculars for astronomy cost? First look for a quality manufacturer. Just because you’ve chosen a good name doesn’t mean you’re draining your pocket. Smaller astronomy binoculars of high quality are usually around or under $25. Mid-sized astronomy binoculars range from $50 to $75 as a rule. Large astronomy binoculars can run from a little over $100 to several hundred dollars. Of course, choosing a high-end pair of binoculars of any size will cost more, but with proper care they can be handed down through generations of users. Keep in mind little things that might be good for your applications, like rubber-coated binoculars for children who bang them around more, or fog-proof lenses if you live in a high humidity area. Cases, lens caps and neck straps are important, too.

Some Suggested Binoculars
The purpose of this guide was to help you understand how to choose the best binoculars for astronomy. But if you trust me, and just want some suggestions… here you go.

For all purpose astronomy binoculars, I’d recommend the Celestron Up-Close and Ultima Series as well as Meade Travel View. Nikkon and Bushnell binoculars in this size range are an investment, and best undertaken after you decide if binocular astronomy and this size is right for you. offers a wide range of these binoculars.

While so much information on binoculars may seem a little confusing at first, just a little study will take you on your way to discovering astronomy binoculars that are perfect for you!

Happy Birthday Johannes Kepler!


December 27 is a day to celebrate the life of astronomer Johannes Kepler, who was born on this date in 1571, and is best known for his three laws of planetary motion. But also, coming up in 2009, The International Year of Astronomy (IYA) will celebrate the work of Kepler as well. Not only did Galileo begin his observations with a telescope almost 400 years ago in 1609, but also in that year Kepler published his book New Astronomy or Astronomia Nova. This was the first published work that documented the scientific method.

Kepler’s primary reason for writing Astronomia Nova was to attempt to calculate the orbit of Mars. Previous astronomers used geometric models to explain the positions of the planets, but Kepler sought for and discovered physical causes for planetary motion. Kepler was the first astronomer to prove that the planets orbited the sun in elliptical paths and he did so with rigorous scientific arguments.

An offshoot of Astronomia Nova was the formulation of concepts that eventually became the first two of Kepler’s Laws:

First Law: The orbit of a planet about the Sun is an ellipse with the Sun’s center of mass at one focus.

Second Law: A line joining a planet and the Sun sweeps out equal areas in equal intervals of time.

And Kepler’s third Law: The squares of the periods of the planets are proportional to the cubes of their semi-major axes.

Kepler was also instrumental in the development of early telescopes. He invented the convex eyepiece, which allowed an expanded field of vision, and discovered a means of determining the magnifying power of lenses. He was the first to explain that the tides are caused by the Moon and the first to suggest that the Sun rotates about its axis. He also was the first to use stellar parallax caused by the Earth’s orbit to try to measure the distance to the stars.

While Kepler remains one of the greatest figures in astronomy, his endeavors were not just limited to this field. He was the first person to develop eyeglasses designed for nearsightedness and farsightedness, the first to investigate the formation of pictures with a pin hole camera, and the first to use planetary cycles to calculate the birth year of Christ. He also formed the basis of integral calculus.

Kepler’s many books provided strong support for Galileo’s discoveries, and Galileo wrote to him, “I thank you because you were the first one, and practically the only one, to have complete faith in my assertions.”

Original News Source: The Writer’s Almanac

Studying Planets With Sunglasses


While finding a planet orbiting another star is incredibly exciting, it’s almost becoming commonplace. The current exoplanet count is up to 270. So now that astronomers know where these exoplanets are located, they are currently devising new techniques in order to study the planets in detail. Using a new method similar to how polarized sunglasses filter away reflected sunlight to reduce glare, an international team of scientists were able to infer the size of an exoplanet’s atmosphere, plus directly trace the planet’s orbit.

Orbiting a dwarf star in the constellation Vulpecula and lying approximately 63 light years from earth, this exoplanet was discovered two years ago. Using this new polarization technique, the astronomers were able to see details about the planet called HD189733b that aren’t possible to observe using other indirect methods. The scientists extracted polarized light to enhance the faint reflected starlight “glare” from the planet, and for the first time, were able to detect the orientation of the planet’s orbit and trace its motion in the sky.

This new technique also indicates that the atmosphere of the planet is quite large, about 30% larger than the opaque body of the planet seen during transits, and probably consists of small particles, perhaps even tiny dust grains or water vapor.

Earlier studies of HD189733b using the Hubble Space Telescope indicated that this world doesn’t have any Earth-sized moons or a discernible ring system. Also, the temperature of its atmosphere is a blazing seven hundred degrees Celsius.

The planet is so close to its parent star that its atmosphere expands from the heat. Until now, astronomers have never seen light reflected from an exoplanet, although they have deduced from other observations that HD189733b probably resembles a “hot Jupiter,” a planet orbiting extremely closely to its parent star. Unlike Jupiter, however, HD189733b orbits its star in a couple of days rather than the 12 years it takes Jupiter to make one orbit of the sun.

“The polarimetric detection of the reflected light from exoplanets opens new and vast opportunities for exploring physical conditions in their atmospheres,” said Professor Svetlana Berdyugina, leader of the group from Zurich’s Institute of Astronomy and Finland’s Tuorla Observatory. “In addition, more can be learned about radii and true masses, and thus the densities of non-transiting planets.”

They discovered that polarization peaks near the moments when half of the planet is illuminated by the star as seen from the earth. Such events occur twice during the orbit, similar to half-moon phases.

Original News Source: Swiss Federal Institute of Technology Press Release