Backyard Astronomy

Backyard astronomy can take as much or as little time as you want to devote to it.  Many people find it rewarding to be able to look up into the night sky and know something about what they see there.

 

Learning the Constellations

Learning to identify some constellations is a good way to start familiarizing yourself with the night sky.  You'll need...

  • A star chart (see www.skymaps.com)
  • A dim flashlight, or a flashlight with a colored filter (so that checking your star chart doesn't cause you to lose your night vision).
  • A dark area with a good view of the sky.

Look over your star chart before you go outside.  Find one or two constellations that have bright stars in them and patterns that are easy to identify.  (The Big Dipper is a good example.  Orion is good, too, but only in the fall and winter.)  Tracing the outline of your target constellations with a highlighter just before you go outside will help you to hold its shape in your mind while you search.

Your hand can be a good tool for measuring distance across the sky.  Hold your hand out to arms length, fingers spread - it will cover about 20O of the sky.  Bringing your fingers together cuts this down to about 10O.  One finger is about 2O.

 

Star Lore

Star lore includes the stories associated with the various constellations and asterisms.  Almost all cultures around the world have some sort of star lore.  Many of these stories may have been an aid to learning and remembering the constellations, as these were important for navigation and agriculture.

Star lore involves both constellations and asterisms.  An asterism is an easily recognized group of stars that may be part of a larger constellation or may contain stars that are part of several constellations.  For example, the Big Dipper is an asterism that is part of Ursa Major (the Great Bear), and the Teapot is part of Sagittarius.  The Summer Triangle, on the other hand, is made up of three stars from three different constellations: Vega (in Lyra, the Harp), Deneb (in Cygnus, the Swan), and Altair (in Aquila, the Eagle).

 

Amateur Astronomy

If you have a pair of decent binoculars or a low-power telescope, you've got better equipment than Galileo ever had!

Many amateur astronomers enjoy taking pictures of the night sky - either with film or with digital cameras.  Set the focus for infinity, open the diaphragm all the way, and set the exposure for 10 seconds.  (Note: a tripod is essential for this, so you don't shake the camera!) 

Star trails will appear if you expose your picture for more than 15 seconds.  These happen when the Earth's rotation cause the stars to move, causing their images to form streaks.  If you want to take a picture of star trails, leave the shutter open for 20 minutes or so.

 

Small Telescopes

A telescope allows you to see more objects (and more interesting objects) in the night sky.  For reflecting telescopes, the bigger the mirror, the more light it gathers and the fainter the objects you'll be able to see.

Useful magnification is limited to about 100x to 200x, due to distortions caused by the atmosphere.  At higher magnifications, these distortions blur whatever you're looking at.

 

Star Charts

Star charts are basically maps of the celestial sphere.  They can be used to locate objects in the sky, and generally give information on the brightness of stars and the time of year that various objects can be seen.

Just like maps of the Earth, many star charts use coordinate grids.

 

Celestial Coordinates

Star charts use coordinate grids, like maps of the Earth.  To avoid confusion, their lines are not called latitude and longitude, but...

  • Declination (dec) - East-west lines that run parallel to the celestial equator.  Declination values go from +90o (north celestial pole) to -90o (south celestial pole).  The celestial equator is 0o declination.
  • Right Ascension (RA) - North-south lines that run from north celestial pole to south celestial pole.  RA values are given in terms of hours, minutes, and seconds - with there being a total of 24 hours of RA.  (Each RA hour is equal to 15o.)  The RA point 0h0m0s is the point where the ecliptic crosses the celestial equator as the Sun moves north (the vernal equinox).

Another system of locating objects in the sky is by using altitude and azimuth.  This system is based on the horizon, not on the celestial sphere.

  • Altitude - An object's angle above the horizon.
  • Azimuth - The angle between North and a point on the horizon directly below the object.

The altitude and azimuth method is less cumbersome, but the values change as objects move across the sky.  An object's declination and right ascension, however, are stable for tens of thousands of years.

 

Planetary Configurations

Planets do not have a stable declination and right ascension because they move against the celestial sphere.  Astronomers use other terms to describe their locations...

  • Conjunction - When the planet is aligned with the Sun, as seen from Earth.
    • Inferior Conjunction - When the planet is between the Earth and the Sun.  (Mercury and Venus.)
    • Superior Conjunction - When the Sun is directly between the Earth and the planet.  (All planets.)
  • Transit - Occurs when a planet passes between us and the Sun, so that we can see it as a dark spot on the Sun.  (Only Mercury and Venus, when seen from Earth.)  2012 Transit of Venus.
  • Opposition - When Earth is directly between the planet and the Sun.  (All but Mercury and Venus.)

Planets at opposition are at their closest to the Earth, appear at their brightest in the sky, and rise at sunset.

Mercury and Venus are easiest to see when they are fartherst from the Sun in the sky.  Since they are closer to the Sun than Earth, there is a limit to how far from the Sun they can appear.  This is called their greatest elongation.

  • Mercury is never more than 28o from the Sun.
  • Venus is never more than 47o from the Sun.

This is why these planets are usually only visible just before sunrise and just after sunset.  Either of these, seen at dawn or dusk, can be referred to as the Morning Star or the Evening Star.

The length of time between two planetary configurations (or moon phases) or the same type is called the synodic period.  This could be the time from one full moon to the next full moon, or from one opoosition of Mars to the next.

 

Your Eyes At Night

You may notice that the longer you stay in dim light, the more sensitive your eyes become - allowing you to see better under the low-light conditions.  This is called dark adaption.  There are two main parts to dark adaption...

  • Your pupils open much wider in the dark, to allow more light to enter.
  • The retina of your eye under go chemical changes that make it about 1 million times more sensitive to light than it would be in full daylight.
    • These changes also make you more sensitive to blue colors, since starlight tend to be more blue and daylight is more yellow.

It takes about 20 minutes for your eyes to become dark-adapted, and this can be undone by just a few seconds of bright light.

You might also notice that it is easier to see faint stars if you look slightly to the side of them (averted vision).  This is because the center of your retina is packed with receptors for seeing fine detail - not for seeing faint light.

 

"Backyard Astronomy: Tips on Observing the Universe" by Sky and Telescope.

Video of the 2012 Transit of Venus - click here.

  

Homework from the Text:

  • Read Essay 1 (pages 63-72).
  • Answer Review Questions #1-10 on page 72.