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Starry Night® Times

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Welcome again to our monthly newsletter with features on exciting celestial events, product reviews, tips & tricks, and a monthly sky calendar. We hope you enjoy it!

   

A Heavenly Choir of Angles, Part II

In the last article, we had discovered that the separation of any two objects in the sky can be described by an angle, if you imagine that the two objects both lie on a circle, and the observer is at the centre of the circle.

A whole circle has 360 degrees; a degree contains 60 arcminutes; an arcminute contains 60 arcseconds. You can only see half the dome of sky at any one time, so if you go all the way across the sky from horizon to zenith (directly overhead) to the opposite horizon, you've measured 180 degrees. From the horizon to the zenith is 90 degrees.

The standard notation uses the symbol ° for degrees, ' for arcminutes, and " for arcseconds.

You can measure the angular size of an object on the sky by measuring the angle between one side of it and the other. Here are some examples of the angular sizes of things in the sky:

The Milky Way almost 150° from the centre in Sagittarius to the edge in Perseus
The Big Dipper about 25° from handle-end to bowl rim
Binocular field of view 7° 30', or 7.5°, for a 7x50 pair
Andromeda Galaxy, M31 over 3° across, although the outer parts are very faint
Telescope field of view between 1° and 2° for a backyard scope and typical eyepiece
The Sun or the full Moon about 32' in diameter, or just over 0.5°
Hercules Cluster M13 about 20' in diameter
Ring Nebula, M57 1' 20" in diameter, or 1.33'
A very thin crescent Moon if it's thinner than 1' you probably can't see it naked-eye
Saturn's rings between 35" and 45" across, depending on distance from Earth

Remember that when you look at something far away, it appears smaller than an identical object that's closer to you. The Sun is far larger than the Moon, but from Earth it looks like it's the same size as the Moon because it's farther away. The Hercules Cluster appears to be a little bit smaller than either of them -- even though it's many times bigger than our entire solar system, containing over a hundred thousand red giant stars!

Last month we looked at a rough method of measuring angular distances and sizes using your outstretched hand. If you want to know the angular size of an object that's too small for that method, you can look at them through a telescope or binoculars and compare them to the size of the field of view.

If you don't know the size of your field of view, look through your telescope at an object whose size you already know. The image below is a simulation of the view through an 8" Orion SkyQuest Dobsonian reflector with its included 25mm Sirius Plossl eyepiece. The Moon appears to be a bit smaller than half the width of the field of view. Since we know that the Moon is approximately half a degree across, we can estimate by eye that the field of view is a bit more than a degree in diameter.

Remember that this is still a rough estimate -- the angular size of the Moon's disc (regardless of what phase it's in) changes a little bit over the course of a month, because its orbit around the Earth isn't perfectly round. Sometimes it's a little bit farther away and looks smaller; sometimes it's closer and looks larger. In a future installment, once we've learned a few more concepts related to measuring the sky, we'll learn some more accurate methods of measuring your field of view.

One warning about terminology: "field of view" (or FOV) is a term that refers to more than one thing. The box an eyepiece comes in usually states the apparent field of view (often abbreviated to AFOV). This is the angle that you will see if you hold the eyepiece up to your eye and look through it by itself. Look at the opposite wall of your room through an eyepiece with a narrow AFOV, and you might see part of your window; look through an eyepiece with a wider AFOV, and you might see the whole window plus a bit of floor and ceiling. The AFOV of an eyepiece is determined by various factors in its optical configuration, and typically ranges from around 50° (for inexpensive eyepieces that come included with telescopes) to over 100° (for high-end specialty eyepieces).

This does not mean that you will see 50° or 100° of sky when you look through a telescope with that eyepiece! When you put the eyepiece on a telescope, you're introducing more optical components into the system. The true field of view (or TFOV) is the width of the bit of sky that you see when you look through a telescope with an eyepiece attached. The same eyepiece on a different scope will have a different TFOV; so will the same scope with a different eyepiece. In the example above where compared the field of view with the width of the Moon, the quantity we were measuring was the TFOV—the field of view of an optical system that includes a certain telescope and a certain eyepiece.

If you have binoculars instead of a telescope, the eyepieces are attached and can't be changed, so you have a fixed system with a fixed FOV. The angle of the FOV will be listed with the other features and specifications of any binocular you buy. In general, just remember that a higher magnification means a narrower FOV -- if you look at a distant object through a binocular that magnifies ten times, the object will fill more of the field than it would if viewed through a binocular that magnifies seven times.

Stay tuned for more practical applications of angular geometry, including coordinate systems on the Earth and in the sky.

Brenda Shaw
Brenda is an avid stargazer who enjoys guiding everyone to the stars, sharing her passion and knowledge with others.

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Spectacular Summer Sights: Shooting Stars

Anyone gazing at the summer night sky for even a short length of time is likely to spot a few "shooting stars" darting across the sky.

Meteors are typically bits of material left behind by comets. They're often no larger than sand grains, and they vaporize as they enter our atmosphere. In general, the Earth encounters richer meteoric activity during the second half of the year. Between August 3 and 15, there are a half-dozen different minor displays that are active.

At its peak around the nights of Aug. 11 and 12, the annual Perseid meteor shower can produce 50 to 100 fast, bright meteors per hour.

This will be a fair-to-good year to watch for the Perseids. A bright gibbous moon, which initially will interfere with observations, will set at around 1:30 a.m., leaving the rest of the night dark for prospective meteor watchers. The only equipment you'll need is your eyes and a modest amount of patience.

Stefan Seip, took this picture over Riedseltz, France. The image shows the Milky Way in the Cepheus and Summer Triangle region. It shows also 6 Perseids and one Non-Perseid meteor. August 12/13, 2005 22:32 - 1:28 UT Photo Details: Camera: Canon EOS 20Da, tracking the stars Lens: Canon EF-S 10-22mm @ 11mm f/4 ISO 1600 Exposure time for the background: 10x 2 minutes

Early morning is best

The main trick is to plan your meteor-watching for the pre-dawn hours. Not only will the moon have set, leaving skies darker, but there are simply more meteors then. This is due to the fact that during the pre-midnight hours we are on the "trailing" side of the Earth, due to our orbital motion through space. So any meteoric particle generally must have an orbital velocity greater than that of the Earth to "catch" us.

However, after midnight when we are turned onto the Earth's "leading" side, any particle that lies along the Earth's orbital path will enter our atmosphere as a meteor.

These objects collide with our atmosphere at speeds of 7 to 45 miles (11 to 72 km) per second, their energy of motion rapidly dissipates in the form of heat, light, and ionization, creating short-lived streaks of light popularly referred to as "shooting stars."

Already begun

The very first forerunners of the Perseid shower began to appear around July 17. Unfortunately, that virtually coincides with a full moon, but even without any interfering moonlight you would only see a few per hour at best.

The numbers will begin to noticeably ramp up during the second week of August. The last Perseid stragglers may still be noted as late as Aug. 24.

To go along with the Perseids, however, there are at least ten other minor meteor displays that are active at various times during July and August. While the hourly rates from these other meteor streams are but a fraction of the numbers produced by the Perseids, combined, overall they provide a wide variety of meteors of differing colors, speeds and trajectories.

Among these are the Southern Delta Aquarids, which reach their peak around July 28 and can produce faint, medium speed meteors; the Alpha Capricornids, which arrive at their maximum a couple of nights later on July 30 and are described as slow, bright, long trailed meteors and the Kappa Cygnids, peaking near Aug. 17 and have been classified as "slow moving and sometimes brilliant."

Earlier this year, Peter Jenniskens of the SETI Institute announced he had identified the probable breakup of a comet from several thousand years ago that may be responsible for the Kappa Cygnids; the asteroid 2008 ED69 may be a fragment from that breakup.

As meager as the individual hourly rates are with the minor displays running from mid-July through the third week of August, collectively they become strikingly augmented with the annual August Perseids. British observational meteor astronomer, Alastair McBeath comments that August is Perseid month, with " ... rising sporadic meteor rates, mild weather overnight, several other minor showers on show and it's vacation time. With the Perseids partly moon-free, all we need are clear skies!"

SPACE.com will provide a complete Perseids viewing guide on Aug. 8.

Joe Rao, SPACE.com Skywatching Columnist
Joe serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for
The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.

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Mars Phoenix Mission: Revelations Abound

The unprecedented success of NASA’s Phoenix Mars Mission has created renewed interest in the Red Planet for many astronomers and planetary enthusiasts. Industry websites are awash with compelling reports and images of the Phoenix Mars Lander’s conquests, as it performs various tasks and analyses on the Martian surface.

This latest mission has given scientists enormous insights into Mars ancient past and will undoubtedly prepare us for continued exploration and possible future habitation. For the time being, however, we earthlings must be content with studying and admiring Mars from afar. Thankfully, modern technology has provided some simple, affordable options for doing that.

From Earth’s perspective, Mars is still visible through August, low on the western horizon during the evening twilight hours. Here are some product ideas from Orion® to optimize your viewing experience…

 

Mars Phoenix Mission data is also available through our award-winning Starry Night® software. To receive the latest files, select File-Update Comets/Asteroids/Satellites from the main menu bar in Starry Night. This update is only available for version 6 users.

For up-to-date news and information about the mission, we recommend the following links:

Albert Kemp
Multi-Channel Commerce Division, Imaginova® Corp.

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Constellation in Focus: Sagittarius

Constellation Map: Sagittarius

M6 and M7, two open clusters, are bright and obvious in Sagittarius, and make for easy binocular objects. Telescopes open up both in rich detail and M6 is seen to be aptly named "The Butterfly Cluster".

NGC 6416 is a small open cluster and NGC 6383 is a dim, wide cluster with nebulosity.

M8 "The Lagoon Nebula" is the brightest nebula after the great Orion nebula. It's actually more massive than M42 but is farther away: 4,500 lightyears distant compared with 1,500 lightyears. M8 is best viewed with a wide-field eyepiece. Less spectacular, but still worth some time, M20 "The Trifid Nebula" is also easily seen in binoculars; a telescope will bring out the dust band that gives the nebula is shape and name. M21 is a small rich open cluster in the same field of view as M20.

M23, excellent in small scopes, is an open cluster seen in binocs, as is M25.

M24 "Delle Caustiche" is a large and lovely "frothy" looking region seen easily in binoculars. It's actually part of the Milky Way and only stands out as a distinct patch because, like M23 and M25, it sits in front of a dark nebula that obscures our line of sight to the core of the galaxy. (By the way, the very center of our galaxy is marked above with a red target symbol.)

M22 is a sweet globular cluster, the third-brightest in the sky. Populated by half a million stars, it's distant by a mere 10,000 lightyears, making it the nearest glob to Earth.

M16 and M17 are two nebulae, the latter in particular a rewarding target. M16, however, is notable for being the location of "The Pillars of Creation" the iconic image produced by the Hubble Space Telescope. M18 "The Black Swan" is a pretty open cluster with about 40 members, surrounded by fainter background stars in the band of the Milky Way.

Sean O'Dwyer
Starry Night® Times Editor

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Astrophoto of the Month

Astrophoto of the Month

The Dumbbell Nebula, Messier 27
Taken by Terry King, Southwick, Sussex, UK. Equipment used: 10" Newtonian, Canon 350D DSLR, Sub's and Dark frames stacked in DSS, and Processed in Photoshop.

   

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  • Send images, following the above guidelines, to photo@starrynight.com (by sending an image you agree to the above terms, including Imaginova®’s right to publish your photos). Please do not send .ZIP files as they will not reach us.

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AUG 2008

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Egypt Pyramid Panorama

Panorama of the Pyramids at Giza, Egypt. Provided by Tom Policano.
   

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SN Pro 4.x and higher users: simply download this archive and extract the two files then place them in your Sky Data/Horizon Panoramas Folder.

Pedro Braganca
Content Director,
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Sky Events
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A guided video tour of celestial events visible this month.

  • Click Here to Download

   

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Tips Tricks
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Tips & Tricks
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Surface Textures

Change it up by adding your own planet surface textures.

In your user’s manual read up on the Orbit Editor for information how you can do this. Here is a link on the internet to get you started:

   • maps.jpl.nasa.gov

Why don’t you share other links with us? Just send us an email!

Pedro Braganca
Content Director,
Starry Night®
   

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Moon Phases

Fri., Aug. 1
New Moon, 6:13 a.m.

As always, New Moons are too near the Sun to be seen. However, sharp-eyed observers may be able to catch a very thin crescent in the western sky after sunset on the day following (Saturday). The second day following (Sunday) should be easier.

Fri., Aug. 8
First Quarter Moon, 4:20 p.m.

The Moon was at "Last Quarter" of the previous cycle on 7/25. Since then it has gone through "New Moon" phase, which marked the end of the previous cycle and the beginning of the current cycle. First Quarter, about a week later, means that the Moon has progressed through about 25 percent of its current cycle.

Sat., Aug. 16
Full Moon - Partial lunar eclipse, 5:17 p.m.
Unfortunately, this eclipse is not visible in North America except very minor portion visible from the northeastern part of the continent. This eclipse is best seen from Africa, the Middle East and parts of western Asia and Russia. The rest of us will just have to wait until the next total lunar eclipse on December 21, 2010. This is the 8th Full Moon of 2008. It is sometimes called the "Dog Days" Moon, due to the so-called "Dog Days" of August, which in turn relate to the rising of the Dog star Sirius just before the Sun in ancient Egypt. To the Micmac People of Canada, this was the Moon "When Young Birds Are Full-Fledged." To the Cheyenne and Lakota Sioux on the American Plains, the 8th Moon was named in honor of ripening cherries, an important supplemental food. To the Ponca it was the "Corn is in Silk Moon." And for the Tlingit on the Northwest Coast, it was the Moon when "All Kinds of Animals Prepare Their Dens." (Moon names courtesy of The Moon Book by Kim Long, Johnson Books, Boulder, 1998.)

Sat., Aug. 23
Last Quarter Moon, 7:50 p.m.

Three quarters of the way through its current cycle, this is also called the Third Quarter Moon. It rises late and sets around mid-day tomorrow.

Sat., Aug. 30
New Moon, 3:58 p.m.

Once every "moonth" (about 29.5 days), the Moon passes the Sun in the sky to start a new orbital cycle. Hence it is a "New Moon." Since it is apparently so close to the Sun in the sky, the Moon cannot be seen at this time, but sharp eyed observers may catch it as a very thin Crescent Moon low in the western sky the following night, or Sunday night in this case.

Observing Highlights

Fri., Aug. 1
Total Solar eclipse, 6:22 a.m.
Unfortunately this total solar eclipse is not observable at all from most of North America. However, a small portion is visible from far northeastern Canada and northern Greenland. The greatest eclipse is at 6:22 a.m. EDT. The best locations are in Europe and Asia. For details, see the NASA Eclipse web site.

Tue., Aug. 12
Perseid Meteor Shower peak, 7:00 a.m.

Timing is to astronomy as location is the real estate -- everything! This year the timing is not great, but it isn't bad. Although the peak is predicted for after sunrise in the East, Perseus is well situated high overhead about two hours earlier. The Waxing Gibbous Moon sets shortly after midnight, so it won't interfere this year. As always, however, best views are from very dark locations, with eyes that are fully dark-adapted.

Thu., Aug. 28
Zodiacal Light, predawn

The Observer's Handbook of the Royal Astronomical Society of Canada (affectionately known as just the "OH") suggests conditions for the next two weeks may provide good chances to observe the faint and illusive "Zodiacal Light" in the eastern sky before dawn. The phenomenon, also sometimes called "False Dawn," appears as a large, cone-shaped region of light rising upward from the eastern horizon to the south. It is thought to be caused by sunlight reflecting off particles along the Ecliptic (the Earth's orbital path). Similar conditions will occur for the evening sky in March.

Planets

Mercury has entered the evening sky, although it remains low in the western twilight all month. It is near Venus and Saturn at times during August, although all are low in the western sky and set within about an hour of sunset.

Venus is an "evening star" for now, but it is quite low in the West at sunset and visible only for a short time thereafter. The current geometry causes it to hug the western horizon for Northern Hemisphere observers. It is near Saturn (which it passes on the 13th) and Mercury (which passes Venus on the 23rd), and not far from increasingly faint Mars.

Mars remains in Leo until it crosses the border into Virgo about the middle of the second week of the month. In the western sky at sunset, it is not bright -- about the equivalent of a second magnitude star. A finder chart or Starry Night are useful in identifying it, and its image in a telescope is too small to reveal any significant detail. On the good side, at the time of this writing all three robot explorers on the surface of Mars -- Opportunity, Spirit and Phoenix -- are alive and well and sending back information!

Jupiter continues to dominate the evening sky, and after Venus sets it has no competition at all. It is well up in the southeastern sky (in Sagittarius) passing through the region of the sky in the general direction of the center of the Milky Way Galaxy. A fabulous sight in a small telescope, Jupiter sets in the southwestern sky several hours after midnight.

Saturn is just about gone, deep in the glow of the setting Sun. Determined observers may still find it for a very short time after sunset, low to the West. Venus passes by on the 13th. After that, Saturn is lost to the Sun until it re-emerges in the morning sky in late September.

Dates

Sat., Aug. 2
Moon-Venus, 11 a.m.

An extremely thin Crescent Moon passes about 2 degrees South of Venus at this time, which unfortunately cannot be seen because of daylight. However, just after sunset, observers with keen eyesight and excellent conditions may be able to catch the fingernail crescent several degrees to the left of pinpoint Venus, very low in the western twilight.

Sun., Aug. 3
Moon-Mars, evening twilight

The Crescent Moon passes about 4 degrees below Mars, low in western evening twilight. Mars is to the upper left of the Moon, whereas Saturn is a bit farther away to the right of the Moon. Saturn is brighter than Mars.

Sun., Aug. 10
Moon passes Antares, 3:00 p.m.

This occurs before moonrise, but the Moon lingers near the star for several hours. Look to the Moon in the southern sky as it gets dark. Antares will be several degrees to the upper right. This is an occultation (eclipse of the star) as viewed from parts of South America.

Wed., Aug. 13
Venus passes Saturn, 3:02 p.m.

Venus passes Saturn by the apparent distance of about a fifth of a degree, less than half the width of the Moon. Unfortunately, the exact moment occurs in broad daylight for North America, but the two are still close in the western twilight after sunset. They are quite low to the horizon, in Leo. Venus, which is considerably the brighter, is just to the left of Saturn. Mercury, which is brighter than Saturn but still much outshone by Venus, is about three degrees to the lower right. Mars, even dimmer than Saturn, is 17 degrees to the upper left of the Venus-Saturn pair. Due to the nearness to the horizon and short period of visibility, this is a difficult observation and requires an unobstructed view to the West. Binoculars are helpful, but be sure not to use them until the Sun is fully down. Starry Night can help you identify each planet.

Fri., Aug. 15
Neptune at Opposition, 3:46 a.m.

Unless you have a telescope and are familiar with the sky, skip this observation. Neptune is a tiny blue dot even in a telescope, but viewing it is a bit of an accomplishment. Several hours before dawn it is in the southwestern sky, in Capricornus. Being opposite the Sun right now, it is closest to the Earth for the year, and brightest. Still, it is several times to dim to be seen with the unaided eye. You will need a star chart, or better yet, Starry Night, to help you pick it out.

Fri., Aug. 15
Mercury passes Saturn, 8:04 p.m.

This is a difficult observation because these planets, along with brighter Venus, are too close to the Sun to be seen easily. However, binoculars could be helpful to show Saturn less than a degree to the upper right of Mercury, both of which are roughly two degrees to the lower right of brilliant Venus, very low in the western twilight a half hour after sunset.

Fri., Aug. 22
Mercury passes Venus, 1:09 a.m.

As Mercury passed Saturn a week ago, today it passes much brighter Venus. This is very low in the western evening twilight. By the actual moment of passage, both planets will have gone down already, but should provide a brief period of visibility just as it gets dark a few hours before. Mercury is about a degree to the lower left of Venus. Don't wait any more than about a half hour after sunset (when the planets will be only about 5 degrees above the horizon), or they will be gone.

Sat., Aug. 23
Moon passes the Pleiades, pre-dawn

The Last Quarter Moon approaches the Pleiades star cluster from the West on Saturday morning. It passes the cluster in daylight, but by Sunday morning it can be seen to the East of the cluster. Look high to the South before dawn.

As always, there's more to explore on NightSky.

Data for this calendar has been derived from a number of sources including the Observer's Handbook 2008 of the Royal Astronomical Society of Canada, Starry Night® software, and others. Only events with a reasonable possibility for Northern Hemisphere observers, or those events with some other significance, are given. All times shown are U.S. Eastern Time.
   

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