06 Sep 2015

2D1S How to Observe

No equipment needed! All observations should be made with the naked

06 Sep 2015
 How to Observe

No equipment needed! All observations should be made with the naked eye, but please do use eyeglasses if you normally require them for long-distance viewing. Telescopes were not in use when the Arab star calendars were made and in use. Therefore, if you have to use binoculars to see a star, you are probably observing too close to the time of sunrise. The only thing you’ll need to follow along with these star calendars is a trusty alarm clock (see below). (However, I won’t discourage you at all from using binoculars or a wide-field telescope to appreciate the majesty of the starfields we’re about to explore!)

Time of night and stellar magnitude. Most of the star calendars of ancient Arabia work by observing stars as they rise or set shortly before the sun rises. (My apologies to the night owls among us!) The star(s) should be almost touching the horizon (within 5-10 angular degrees or so) to be in the proper position. Historically, the rain stars (anwa’ ) and the lunar stations were typically observed at a time of night called ghalas, when the darkness of the night mixes with the light of dawn in the tracts of the horizon. You will find that the period of ghalas occurs between 75 and 45 minutes before sunrise, depending upon your observing latitude. (Note that the time of sunrise will change throughout the year as we progress from fall to winter, spring and summer. Your local times of sunrise throughout the year can be found on a website like timeanddate.com.) The proper mixture of night and dawning light in which the brighter stars are still visible represents a precise period of the night, but not an exact time. Minutes were not in use prior to the European Middle Ages, so don’t get caught up in trying to find an exact time.

In practice, the proper timing for this depends upon the brightness (magnitude) of the star and whether they are rising or setting. The brightest stars are visible just before sunrise, but the dimmer ones may require observation more than an hour before sunrise. Dim stars will also be more difficult to see in locations where city pollution creates a layer of smog low on the horizon. Note that stars that rise in the east before the sun will be in a brighter patch of sky than those setting in the west. Therefore, the same star may require an earlier observation time when it is rising ahead of the sun than when it is setting on the opposite side of the sky.

If your schedule does not permit observation of these stars during the wee hours of the morning, there is still hope for you! The stars that set in the morning can usually be observed at different parts of the sky earlier in the evening on the same day. In the descriptions for each star or star grouping, I will provide alternate evening observation times. Note that their orientations will be different during the evening observation times.

Time of year, latitude and local horizon. First things first: The Arabs were not trying to calibrate an atomic clock by the movement of the stars. Just as the time of night is specific but not exact, so were the timings of a star’s rising or setting. In fact, many of the star groupings required a small period of time for all of them to go through the process of setting or rising. The point is that the stellar settings and risings designated a time of year, not necessarily a specific day, though the latter was made to be the case as mathematical astronomy developed.

The seasonal timings for the setting or rising of stars during the galas of night are given for the latitude of Tucson, Arizona (32.2°N). Longitude has no bearing on stellar positions, so Tucson sees very nearly the same sky that the northern end of the Arabian Desert sees. (Hence, “Two Deserts, One Sky!”) The sky in Tucson appears like it does in the modern-day locations of some of the early Islamic centers of learning: Cairo (30.0°), Damascus (33.5°), Baghdad (33.3°), Kufa (32.0°), Basra (30.5°), and Isfahan (32.7°).

At latitudes above this (e.g., most of North America and Europe), pre-dawn settings or risings for the same star will come slightly later in the year. How much later depends upon how far away the star is from the celestial equator. Stars on the equator will set on about the same day, whereas stars that are quite far from the celestial equator may set a few weeks later. Conversely, at latitudes below that of Tucson, pre-dawn settings will come slightly earlier in the year. The Arabian Desert stretches some 1300 miles from western Iraq at a latitude of about 33.5 degrees down to Yemen at a latitude of about 15.5 degrees. This means that stars will set for much of the Arabian Desert slightly earlier in the year than they will for our chosen standard location of Tucson. Indeed, many of the recorded timings for the setting stars occur 2-3 weeks earlier than we will experience in this next year, but there is more to the story (see precession, below).

Your local horizons will also affect how you observe the stars in the Arab star calendars. If you can see all the way down to the western (for setting stars) or eastern (for rising stars) horizons, then you are fortunate! Many of us have obstructions like buildings, trees or mountains that lie in the way of our seeing a star’s true moment of setting. The good news is that the requirement of observing during galas (see above) moves the setting and rising positions up the sky a bit due to the horizon glow, about 5-10 angular degrees. (If you hold your hand at arm’s length, 5° is roughly the width of your three middle fingers, and 10° is roughly the width of your fist.) Be aware that if you live in or close to a city that throws lots of light into the sky, you will likely need to make your observation time earlier so the stars are higher in the sky, avoiding the greatest levels of sky glow. This also applies if you live in a dark location, but a large city makes your western horizon glow.

Precession of the equinoxes. The Earth spins like a top about its axis, but its axis also wobbles slowly like a top. This means that the axis of the earth carves out a giant circle when projected against the starry sky. This projection of the Earth’s axis completes a full revolution of this circle once every 25,700 years. Today, the projection of the Earth’s axis onto the sky (the celestial pole) is very close to Polaris, which we know as the North Star. In the early days of Islam, almost 1400 years ago, there was no North Star because the celestial pole was about equidistant between Polaris and Kochab, one of the end stars in the Little Dipper. This shift in the orientation of the Earth against the sky means that most of the recorded timings for the star calendars are about 2-3 weeks earlier than we would observe them today.

Astrophotography. Why not take images of the stellar settings and risings over time to record your observations? All you’ll need is a camera that is capable of longer exposures (up to 30 seconds should be sufficient) and a tripod to keep the image steady. You’ll need to experiment with exposure times to make sure you get the faint star light without being overrun by the dawn light from the sky itself.

Have fun! There is something special about connecting with the sky using just your “naked” eyes. Naked-eye astronomical observation is something of a lost art these days because many people want to see greater detail through binoculars or a telescope. Get outside a little bit earlier than your expected observation window so you can get comfortable and let your eyes adjust to the darkness. Locate the stars identified in the blog post for that week, and try to see those stars through the eyes of an Arabian nomad, or perhaps a goat herder or poet.

When you’ve had some time to get in the mood, watch closely. For setting stars, note the last time you can see the star or star grouping before it is swallowed up by the light of dawn. Does the star reach the horizon before it is lost in the light? Does is set before it is consumed by the light? Try to pin down the day when it is just barely visible in the light of dawn at the moment it sets. For rising stars, what is the first day when you can see the star rising just before it gets swallowed up by the sun’s light? Were you able to see enough of the surrounding stars to make sure you saw the right star from the calendar?

You can make a modern star calendar of your own, too! Is there a star that sets or rises when your favorite holiday begins? When your school year begins or ends? When sports playoffs occur? Have fun adapting these star calendars to your modern life!

Leave a comment
More Posts