An artist's portrayal of the symbiotic recurrent nova RS Ophiuchi.
Its spectrum is nearly identical to that recorded in mid-March during
the surprising outburst of the variable star V407 Cygni.

Catch a Star's Unprecedented Eruption

407 Cygni used to be a rather mundane variable star, known only to small elite of variable-star observers, and typically ranged in brightness between 12th and 14th magnitude. But two weeks ago it seized the attention of professional and amateur astronomers worldwide, who'll be keeping a close eye on it from now on.

Japanese amateurs K. Nishiyama and F. Kabashima, nova hunters working the galactic plane, first sounded the alert on March 11th when they announced what appeared to an 8th-magnitude nova near Deneb in Cygnus. Observers quickly realized that this wasn't a stellar debut, but the unexpected eruption of a known faint variable. So what caused a well-behaved star to suddenly erupt violently?

V407 Cygni is a symbiotic variable — a type of close, interacting binary generally containing a red giant (usually an M-type star) and a hotter, smaller white dwarf. They orbit each other inside a shared nebulosity. Typically the red giant transfers matter to the white dwarf via part of its its stellar wind. The rest of the wind gets heated and ionized by the radiation from hot white dwarf, giving rise to the symbiotic nebula.

Such systems are complex. They can vary periodically due to the binary motion, the red giant can vary due to pulsation or the formation of large starspots, or the stars may be partly obscured by circumstellar dust clouds coming and going. The white dwarf and its surroundings may shine more or less constantly as the dwarf accretes and heats gas from the red giant, or the accretion disk around the dwarf may brighten and fade. The mass finally striking the dwarf's surface can result in flickering and quasi-periodic oscillations. If there is a sudden increase in the rate of accretion, or the material in the accretion disk reaches a point of instability and crashes down onto the surface of the white dwarf, the symbiotic system may undergo a brief, nova-like eruption in miniature.

In about 20% of symbiotic binaries, the giant is a Mira-type (pulsing) variable and the paired stars reside in a much dustier envelope. V407 Cygni is one such example. It typically cycles in brightness by a few magnitudes due mainly to the giant's pulsations and was never before witnessed in a nova-like outburst. So no one expected to find it suddenly glowing nearly 100 times brighter than before.

That was just the beginning of the story. Spectra of the system taken on March 13th were different from any recorded for this star or any other symbiotic Mira variable. The giant's normal absorption spectrum was completely overwhelmed by blue continuum radiation from the erupting white dwarf. Emission lines in the spectra revealed two distinct sources: the relatively slow ionized wind of the Mira star, and the rapidly expanding ejecta from the outburst.

Some binary systems, called symbiotic recurrent novae, do exhibit this kind of spasmodic activity. A well-known example is RS Ophiuchi, whose outburst spectrum looks remarkably like that just recorded for V407 Cygni. So perhaps V407 Cygni has joined this rare class.

If that weren't enough, the story took another twist on March 19th. That's when the Fermi Gamma-ray Space Telescope may have detected the star in gamma rays — something never before observed in a symbiotic system. The gamma rays could be caused by shock-driven acceleration of the ejected material and its capture by strong magnetic fields within the system.

An Amateur's Mercury Odyssey

Mercury from Mount Wilson and from Messenger
At left is a composite of 40 near-infrared images of Mercury taken in 1998 with a video camera attached to the 60-inch telescope on Mount Wilson (and, at center, as reprocessed in 2007). Compare them to the nearly identical — but much more detailed — view acquired last year by the Messenger spacecraft. Among the many features in common is the very bright spot above center.

Today amateurs routinely employ stacking — selecting and then combining the very sharpest images to yield the best possible detail. Programs like Registax do this almost automatically. But Dantowitz first had to digitize the analog video and then painstakingly find the best frames by hand. "Stacking" as we know it today hadn't been invented yet. The team called its technique "selective image reconstruction."

Fortunately, all that effort paid off. The resulting composite view showed never-before-seen details on a side of Mercury that had been totally missed during Mariner 10's flybys in the mid-1970s. Particularly intriguing was a very bright 100-mile-wide spot, located at 35°N, 300°W, that the team imagined to be some kind of impact feature. The three amateurs published a tidy article in the Astronomical Journal's May 2000 issue, and that should have been the end of the story.

But as the years went by, Dantowitz kept wondering about the nature of that bright feature. In 2008, knowing that NASA's Messenger spacecraft would soon reveal the true nature of his find — and the entire planet, for that matter — he petitioned the International Astronomical Union to have the bright spot named for American composer Aaron Copland. (The IAU names craters on Mercury after artists, musicians, painters, and authors.)

"Copland wrote Fanfare for the Common Man, one of the most recognizable pieces of 20th-century American classical works," Dantowitz noted in his petition. "This Mercury feature was discovered by using common, off-the-shelf commercial video equipment installed by an elementary-school science teacher doing research at a professional observatory. A true discovery that is a 'Fanfare for the Common Man'!"


Mercury from Messenger
The Messenger spacecraft captured this false-color view of Mercury during its third flyby of the planet on September 29, 2009. The bright spot seen from Earth doesn't look like other craters, and its origin remains unclear. The IAU has assigned the name Copland to the large basin just to the spot's east (arrowed). Click on the image for a larger view.

Last year Messenger did reveal an intriguing bright spot exactly where the Mount Wilson team had spotted it more than a decade earlier. But it's very strange — probably volcanic in origin — so the IAU's planetary moniker-makers and Dantowitz agreed to assign Copland to a true impact basin, 129 miles (208 km) across, adjacent to the mysterious white spot. The naming was announced last week on the Messenger website.

"I finally feel happy about the Mercury work!" Dantowitz exults. Revealing an unseen side of the planet, armed with $100 videocams and sheer determination, was a real triumph. "If only I had then the cameras and computers that I have now," he muses.

The March Equinox Explained

The March equinox will occur on March 20 in 2010, marking the beginning of spring in the northern hemisphere and fall (autumn) in the southern hemisphere from an astronomical viewpoint. The March equinox will occur at 17:32 (or 5:32pm) at Coordinated Universal Time (UTC) on this date.

Twice a year, around March 20 or 21 and September 22 or 23, the sun shines directly on the equator and the length of day and night are nearly equal in all parts of the world. These two days are known as the March(vernal or spring in the northern hemisphere) equinox and the September equinox.

To find the March equinox date in other time zones or other years, please use the Seasons Calculator.

What does equinox mean?

The word “equinox” derives from the Latin words meaning “equal night” and refers to the time when the sun crosses the equator. At such times, day and night are everywhere of nearly equal length everywhere in the world.

It is important to note that while the March equinox marks the beginning of spring in the northern hemisphere, it is the start of autumn in many parts of the southern hemisphere.

March Equinox Explained

The March equinox is the movement when the sun crosses the true celestial equator – or the line in the sky above the earth’s equator – from south to north, around March 20 (or March 21) of each year. At that time, day and night are balanced to nearly 12 hours each all over the world and the earth’s axis of rotation is perpendicular to the line connecting the centers of the earth and the sun.

In gyroscopic motion, the earth’s rotational axis migrates in a slow circle based as a consequence of the moon’s pull on a nonspherical earth. This nearly uniform motion causes the position of the equinoxes to move backwards along the ecliptic in a period of about 25,725 years.

Nearly Equal?

During the equinox, the length of night and day across the world is nearly, but not entirely, equal. This is because the day is slightly longer in places that are further away from the equator, and because the sun takes longer to rise and set in these locations. Furthermore, the sun takes longer to rise and set farther from the equator because it does not set straight down - it moves in a horizontal direction.

Moreover, there is an atmospheric refraction that causes the sun's disk to appear higher in the sky than it would if earth had no atmosphere. timeanddate.com has a more detailed explanation on this topic. timeanddate.com has more information on why day and night are not exactly of equal length during the equinoxes.

During the March equinox, the length of daylight is about 12 hours and eight to nine minutes in areas that are about 30 degrees north or south of the equator, while areas that are 60 degrees north or south of the equator observe daylight for about 12 hours and 16 minutes. Many regions around the equator have a daylight length about 12 hours and six-and-a-half minutes during the March equinox.

Moreover, one day does not last for the exact same 24 hours across the world and due to time zone differences, there could be a small difference in the daylight length between a far-eastern and far-western location on the same latitude, as the sun moves further north during 24 hours. For more information, find out the length of day in a particular city. Select a location in the drop-down menu below to find out the length of day around the time of the March equinox.

Vernal Equinox vs. Autumnal Equinox

The vernal equinox occurs in the spring while the autumnal equinox occurs during fall (autumn). These terms are derivatives of Latin. It is important to note that the northern hemisphere’s vernal equinox is in March while its autumnal equinox is in September. In contrast, the southern hemisphere’s vernal equinox is in September and its autumnal equinox is in March.

This distinction reflects the seasonal differences when comparing the two hemispheres. timeanddate.com refers to the two equinoxes simply as the March and September equinoxes to avoid false assumptions that spring is in March and fall (autumn) is in September worldwide. This is simply not the case.

Historical Fact

A Greek astronomer and mathematician named Hipparchus (ca. 190-ca.120 BCE) was attributed by various sources to have discovered the precession of the equinoxes, the slow movement among the stars of the two opposite places where the sun crosses the celestial equator. Hipparchus made observations of the equinox and solstice. However, the difference between the sidereal and tropical years (the precession equivalent) was known to Aristarchus of Samos (around 280 BCE) prior to this.

Astronomers use the spring equinoctial point to define their frame of reference, and the movement of this point implies that the measured position of a star varies with the date of measurement. Hipparchus also compiled a star catalogue, but this has been lost.

March Equinox across Cultures

In the northern hemisphere the March equinox marks the start of spring and has long been celebrated as a time of rebirth. Many cultures and religions celebrate or observe holidays and festivals around the time of the March equinox, such as the Easter holiday period.

The astronomical Persian calendar begins its New Year on the day when the March equinox occurs before apparent noon (the midpoint of the day, sundial time, not clock time) in Tehran. The start of the New Year is postponed to the next day if the equinox is after noon.

Reflections from Space: Spot Iridium Flares

Reflections from Space: Spot Iridium Flares

Periodically, I'll receive an e-mail from a reader inquiring describing a very unusual sight that they have seen in the sky. A typical inquiry might read something like this:

"I happened to be outside the other night when I caught sight of an ordinary-looking star that suddenly became super-bright. My first thought was that perhaps I was witnessing something akin to a supernova . . . the death of a star."

The person goes on to explain how the flare died away seconds later, as quickly and mysteriously as it came.

If you have ever seen such a sight, then in all likelihood you've witnessed an "Iridium flare," caused by one in a fleet of satellites that have been put into Earth orbit since the late 1990's; satellites that can briefly appear to flare to incredible brilliance.

And you can spot them, too, if you first find out when they're likely to occur.

Space mirrors

An Iridium communication satellite's Main Mission Antenna is a silver-coated Teflon antenna array that mimics near-perfect mirrors and are angled at 40-degrees away from the axis of the body of the satellites. This can provide a specular (direct) reflection of the Suns disk, periodically causing a dazzling glint of reflected sunlight from their 484 mi (780 km) orbits.

At the Earth's surface, the specular reflection is probably less than 50 miles wide, so each flare can only be viewed from a fairly small area.

Iridium satellites normally traverse the sky on the edge of visibility, at +6 magnitude. On this astronomer's scale, smaller numbers represent brighter objects. Venus outshines all stars and planets reaching a magnitude of -4.8 at its very brightest.

Iridium satellites can provide reflective flares of magnitude -8. That's almost 20 times brighter than Venus, based on how the brightness scale works. The flares can last anywhere from 5 to 20 seconds before the satellite once again becomes almost invisible to the naked eye.

In fact, it is even possible to see such flares during the daytime, if you know exactly where to look.

Where to look

If you wish to see such brief flares for yourself, you will first have to know your exact latitude, longitude and local time zone. Then, log on to a web page that will tell you when the next Iridium flares can be seen. One such site is Heavens Above, hosted by the German Aerospace Center.

This site and others also provide viewing information for the International Space Station, the Hubble Space Telescope and other satellites.

A bit of caution: Not all Iridiums flare according to the predicted schedules. Some of the Iridium satellites are either tumbling or otherwise not operational so their future movements cannot be reliably predicted. A fully operational satellite should be orbiting the Earth at 14.34 revolutions per day.

You may wonder why are the satellites called "Iridium?" It has absolutely nothing to do with the metallic element that occurs in platinum ores. Originally, it was conceived that a total of 77 Iridium communication satellites would be placed into Earth orbit. Since the atomic number for Iridium is 77, a satellite constellation's name was conceived.

In reality, a total of 88 satellites were launched between May 5, 1997 and June 11, 1999. An additional five more were launched on February 11, 2002. Typically, the expected lifetime of a satellite is 5 to 8 years.

• How to Spot Satellites


• Sky Calendar & Moon Phases


• Astrophotography 101

Zodiacal Light in the Evening

Zodiacal Light in the Evening

Have you ever seen the zodiacal light? This huge pearly pyramid is on its best display in the Northern Hemisphere on moonless evenings from February through April. All you need is a location far from artificial lights (at least 40 miles from a small city and 80 miles from a major metropolis) that also has an unobstructed western horizon.

Go out an hour after sunset and look to the west. Even though the Sun is now far below the horizon, a huge dome of light marks the spot where it disappeared. As this light fades and shrinks down to the horizon, another glow will be unmasked; a tall, leftward-slanting pyramid of light. It follows the path of ecliptic, running left of Aries and then between the Hyades and Pleaides, the sky's most spectacular star clusters.

Aside from its shape, you might think it was just part of the twilight, but it will linger long after the rest of the sky is fully dark. The first time I saw the zodiacal light, I thought it was light pollution, but light pollution forms a low band along the horizon. It's amazingly brighter — even brighter than the Milky Way — and once you've seen it, you'll never again have trouble recognizing it.

What are you seeing? The zodiacal light is the combined glow of countless tiny particles (debris from comets and asteroid collisions) that orbit the Sun.