Celebrate Astronomy Day: Tips for a Budding Skywatcher

May 6 is Astronomy Day, which was started in 1973 to get people interested and involved in the hobby. 

Traditionally, it has been held in late April or early May on the Saturday closest to the First Quarter Moon.  Since First Quarter is on Friday, May 5, tradition dictates that Astronomy Day is to be held the following day.  Many planetariums and astronomy clubs nationwide will be marking the event with lectures, demonstrations and of course, outdoor observing sessions. 

There is much to be seen now, not only on Astronomy Day, but during the upcoming week as well.

THE MOON: Most might think that the best time to look at the Moon is when it's full, but that's probably the worst time to look at it.  When the Moon is full it tends to be overly bright as well as flat and one-dimensional in appearance. 

In contrast, around the interval when the Moon is just past First Quarter phase (as will be the case this weekend) we get the best views of the lunar landscape right along the sunrise-sunset line or terminator.  It's here that craters, mountains, and clefts appear to stand out in sharp relief. In addition, a half Moon offers more viewing comfort to the eye as opposed to a full Moon. In fact, a half Moon is heavily shadowed, even on its illuminated half. 

Conversely, a Full Moon is completely illuminated.  Especially right around the center of a Full Moon, the Sun shines straight down even into all the microscopic crevices and except for perhaps around its immediate edges, there are no shadows at all.  This is why the Full Moon appears so much brighter than a half Moon.  Some may think a half Moon shines half as bright as a full Moon, but in reality it's only 1/11 as bright!

PLANETS:  As we noted last week, there are three bright planets that are currently visible in our evening sky: Mars, Saturn and Jupiter.  We have left Mars too far behind in our orbit to see hardly any features on it, but Saturn and Jupiter provide a smorgasbord of marvelous sights for telescopes. 

Saturn, which is more than halfway up in the south-southwest sky as darkness falls, is always the showstopper, especially for anyone who has never seen its glorious ring system through a telescope.  For many they seem to glow with a sort of mysterious inner light.  It's of course because they are illuminated by the Sun, and they're not solid disks, but rather, composed of countless particles of matter.

Jupiter, which arrived at opposition to the Sun on May 4, is low in the east-southeast at dusk and shines brilliantly all through the night; reaching its highest point in the south right around 1:00 a.m. In telescopes it displays a disk that abounds with detail.  Try looking not just for the few easy dark belts and light zones, but also for dark festoons and garlands in front of the zones and light rifts in front of the belts, as well as for spots and other features. 

In recent months, a new reddish spot has appeared, officially known as "Oval BA," but "Red Jr." might be better. It's about half the size of the famous Great Red Spot and almost exactly the same color.

DISINTEGRATING COMET: This week Comet Schwassmann-Wachmann 3 will be making its closest pass to the Earth.

During the autumn of 1995, this comet suddenly and unexpectedly fractured into several pieces, and in the process became many hundreds of times brighter than expected, even becoming barely visible to the unaided eye.  Now, again en route toward the Sun, dozens of separate pieces have been sighted with the biggest of these (fragment "C") due to pass within 7.3 million miles of Earth on May 12 - closer than any comet since 1983.  For a number of nights around this date it just might again verge on naked-eye visibility. 

The comet is conveniently visible during the evening hours and will quickly track from a spot south of the bright star Vega on May 7, to the north of Albereo (in Cygnus) on May 9, to south and west of the Great Square of Pegasus by May 17.  

SPACE STATION:Also during this week, the International Space Station (ISS) will be passing over many North American cities on a general northwest-to-southeast trajectory.   The ISS is by far the biggest and brightest of all the man-made objects orbiting the Earth.  The length of its solar arrays measures 240-feet in width.  The Station itself is 146-feet in length.  It stands 90-feet high and weighs 412,000 pounds (206-tons). 

Circling the Earth at an average altitude of 240 miles and at a speed of 18,000 m.p.h., it can appear to move as fast as a high-flying jet airliner, sometimes taking up to five minutes to cross the sky.  It can easily be confused with aircraft lights.  Nominally, its visual magnitude from the ground can reach near -1 (approaching the brightness of Sirius, the brightest star), though at its very brightest it can sometimes appear to rival even Jupiter in brilliance. 

So what is the viewing schedule for your particular hometown? You can easily find out by visiting one of these three popular web sites:

Chris Peat's Heavens Above (http://www.heavens-above.com/),

Science@NASA's J-Pass (http://science.nasa.gov/Realtime/JPass/) or

NASA's SkyWatch (http://spaceflight.nasa.gov/realdata/sightings/).

Each will ask for your zip code or city, and respond with a list of suggested spotting times. Predictions computed a few days ahead of time are usually accurate within a few minutes. However, they can change due to the slow decay of the space station's orbit and periodic reboosts to higher altitudes. Check frequently for updates.

SHOOTING STARS: The Eta Aquarid meteor shower is predicted to peak on May 6.  Early that morning, the Moon sets around 3:00 a.m. local daylight time, leaving just over an hour of darkness before the first light of dawn.  Each morning thereafter the Moon sets about a half-hour later. 

What is especially interesting is that Halley's Comet apparently shed the meteoroids that produce this particular meteor display.  The orbits of the Earth and the comet lie rather close to each other in early May, so at this time of the year we are running through the debris shed by Halley from its previous visits.

Under ideal conditions, when the radiant (the emanation point of these meteors) stands overhead in a dark sky, about 60 very swift streaks of light can be seen darting across the sky each hour.  But the radiant is at the Water Jar of Aquarius, which never gets very high as seen from mid-northern latitudes, so the observed rates are usually far fewer.  On the other hand, Southern Hemisphere observers, such as in Australia, consider this to be the best meteor shower of the year. 

If you want to watch these heavenly streakers, just bundle up on a lawn lounge and start looking up a few hours before sunrise.  When you see a streak, mentally run it backwards across the sky.  Do the same with the second and third and note where their paths cross. This is the radiant and identifies a meteor shower.  A streak that fails this test is an interloper and does not belong to the Eta Aquarids.  The only dangers in meteor watching are getting cold and falling asleep.

Basic Sky Guides

  • Full Moon Fever
  • Astrophotography 101
  • Sky Calendar & Moon Phases
  • 10 Steps to Rewarding Stargazing
  • Understanding the Ecliptic and the Zodiac
  • False Dawn: All about the Zodiacal Light
  • Reading Weather in the Sun, Moon and Stars
  • How and Why the Night Sky Changes with the Seasons
  • Night Sky Main Page: More Skywatching News & Features

Joe Rao 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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1 AU, or astronomical unit, is the distance from the Sun to Earth, or about 93 million miles.

Magnitude is the standard by which astronomers measure the apparent brightness of objects that appear in the sky. The lower the number, the brighter the object. The brightest stars in the sky are categorized as zero or first magnitude. Negative magnitudes are reserved for the most brilliant objects: the brightest star is Sirius (-1.4); the full Moon is -12.7; the Sun is -26.7. The faintest stars visible under dark skies are around +6.

Degrees measure apparent sizes of objects or distances in the sky, as seen from our vantage point. The Moon is one-half degree in width. The width of your fist held at arm's length is about 10 degrees. The distance from the horizon to the overhead point (called the zenith) is equal to 90 degrees.

Declination is the angular distance measured in degrees, of a celestial body north or south of the celestial equator. If, for an example, a certain star is said to have a declination of +20 degrees, it is located 20 degrees north of the celestial equator. Declination is to a celestial globe as latitude is to a terrestrial globe.

Arc seconds are sometimes used to define the measurement of a sky object's angular diameter. One degree is equal to 60 arc minutes. One arc minute is equal to 60 arc seconds. The Moon appears (on average), one half-degree across, or 30 arc minutes, or 1800 arc seconds. If the disk of Mars is 20 arc seconds across, we can also say that it is 1/90 the apparent width of the Moon (since 1800 divided by 20 equals 90).

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Joe Rao
Skywatching Columnist

Joe Rao is Space.com's skywatching columnist, as well as a veteran meteorologist and eclipse chaser who also serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmers' Almanac and other publications. Joe is an 8-time Emmy-nominated meteorologist who served the Putnam Valley region of New York for over 21 years. You can find him on Twitter and YouTube tracking lunar and solar eclipses, meteor showers and more. To find out Joe's latest project, visit him on Twitter.