NavList:

Ahoy Bruce,

You wrote:
" To the west,there were three stars in a curving upward arc; Denebola ,Mars and Saturn? Arcturus ?"

You're definitely seeing Arcturus. Do you remember the trick (and mnemonic) to "follow the arc to Arcturus". If you extend the "sweep" or "arc" of the Big Dipper's handle down to the west (away from the bowl), the very bright star you reach is Arcturus. It's due west about 35 degrees high in the early evening in our latitude these days. If you continue on more or less that same path, and about the same distance again, you will reach Spica rather low in the southwest. Spica is only a little less bright than Arcturus but in mid-northern latitudes it's usually lower in the sky so even fainter. However before you hit Spica, for this year, you'll reach Saturn which is above Spica but relatively close to it (about four degrees higher and nearly the same brightness). You MAY also be seeing Denebola and Mars, but at 9:30 when the sky is just dark enough to see them, they're already rather low in the west. Denebola is not bright at all. Mars is still fairly bright and it's to the lower right of Saturn in the evening in our latitude. Saturn, Mars, and the point of sunset are all in a row. For the planets and Sun and Moon, it's very useful to remember that they all lie close to the ecliptic (the great circle of the Sun's path during the year which runs through the zodiac constellations). So let's say you have found Saturn an hour after sunset and you're looking for another planet. If you point your hand at the location where the Sun set and then sweep across the sky through Saturn, your hand is tracing out the ecliptic across the sky. Anytime you're having trouble guessing a star's identity, try to determine if it's near the ecliptic. If so, it's probably just a planet that you've lost track of (you spend a few months away from sky-watching and the planets will shift around enough to confuse the best observer).

And can you find Cassiopeia? It's a "W" of second magnitude and somewhat fainter stars. Cassiopeia and the Big Dipper are about the same distance from the North Star on opposite sides of it. So in the late evening at this time of year when you face the North Star, you'll find the Big Dipper to the left at about the same height (as Polaris), and you'll find Cassiopeia to the right at about the same height and the same distance from the North Star. In that orientation it is a "W" on its side (like an angular version of the number "3"). It's a fairly compact constellation, about half the size of the Big Dipper. Its stars are somewhat fainter than the stars of the Big Dipper

After Arcturus, Spica, Antares, and the stars of the summer triangle, Vega, Deneb, and Altair, the next band of the sky (often called the "fall sky" since those stars will be twilight stars in a few months) is rather barren. You can try learning the "Great Square" of Pegasus, and you may be able recognize the little flattened triangle of Aries after a while, but there are very few bright stars. The zodiac constellations Capricornus and Pisces are very faint. One exception is the star Fomalhaut which sits all by itself low in the south at the right time of night (around 2am right now you can see it low in the southeast). While the fall sky disappoints, the winter stars aren't far behind. Orion of course is easy. Just remember Betelgeuse is above the belt and Rigel below. Orion is surrounded by a huge "ring" of bright stars: Aldebaran, Capella, Castor, Pollux, Procyon, and Sirius (conveniently, they're in alphabetical order if you go clockwise around Orion). Throw in Regulus in the Spring sky and maybe two or three fainter ones, and that's all the stars you'll ever need for celestial navigation. You'll also discover that it becomes relatively much easier to fill in the fainter constellations once you know those principal bright stars. There are more in the southern hemisphere, but you can worry about that when you go there!

If you want an analog device to learn from, any common planisphere can help a bit. You perhaps had one as a kid. It's a little disk with a star map on it that turns inside a cardboard frame with a sort of oval window representing the sky as seen from some latitude. They're sold at many planetariums and science museums. You can easily order one online: go to amazon.com and search on "planisphere". There are several nice options.

Of course planispheres are nice toys, but they have been almost entirely replaced by computer software in a category usually called "planetarium software". There are nearly endless free or inexpensive software solutions for all sorts of platforms. If you have not yet, you should download a copy of "Stellarium" from stellarium.org. It's quite nice visually, and it's an excellent tool for learning the constellations and many other aspects of astronomy. It's free, but it does have a few small quirks (or bugs, depending on your point of view). It includes a nice option for adding a grid of altitude and azimuth lines on the sky so you can visually read off the coordinates for your evening observing. You can also focus on any star (click on it) and it will display accurate coordinates in various systems including altitude and azimuth. There are similar "planetarium programs" on other platforms. As I've described again recently, Google "Sky Map" is BY FAR the best way to learn the stars these days. Of course, this only helps if you have an Android smartphone (there are good options on the iPhone/iOS platform, too). A huge advantage of any "planetarium software" compared to a traditional planisphere is that the planets and Sun and Moon are automatically included. A disadvantage in some is that objects which are quite invisible to the casual observer are included for a "gee whiz" factor. For example, "Sky Map" will show you how to find Jupiter and Saturn which are bright naked eye objects but it will also show you Neptune, even though you can't see that planet without a telescope.

There's a small, but very convenient and accurate web page at the US Naval Observatory web site which will calculate altitudes and azimuth for any location and time:
http://www.usno.navy.mil/USNO/astronomical-applications/data-services/cel-nav-data
Unlike the "planetarium programs" above, this has no graphics, no star maps. It's a plain text table of data. And incidentally, the software you're seeing here was created decades ago and merely "wrapped" in a web page interface a few years back. Since it was designed with celestial navigators in mind, the stars listed are only the "traditional" navigation stars. This is the list that became the navigators's standard list in 1953. There's nothing sacred about this list of 57 (really 57+1... Polaris is counted as a special case), but since celestial navigation has fallen completely into obsolescence, it is unlikely that any other list will ever replace it, and in that sense it is "the" list (there was a somewhat different list available to the Apollo astronauts but it died with the Apollo program). The output from this program (web page) includes the "Hc" or computed altitude and "Zn" or computed true azimuth. You can calculate for any observing time and location (just remember that the input time and date are UT/GMT so if it's 6pm Eastern Time on July 23, and you want data for 9:30pm, that's equivalent to 01:30 on July 24 Greenwich date). You can ignore for now most of the other data listed. Unfortunately, the table is not set up to provide a magnitude cutoff nor does it indicate magnitudes so rather faint stars are listed along with the very brightest. Also note that some of the star names are abbreviated for the purposes of this table. The star Zubenelgenubi is a good example of both of these issues. It's a faint star between Spica and Antares, and really you could navigate by the stars for your whole life and never have a need to use it, but it's included with the same prominence as Vega and Arcturus. It's abbreviated on this site as "Zubenelg". A simple rule of thumb for the stars on that list: if the name doesn't ring a bell, like, for example "Alphecca", then it's probably fainter and not worth worrying about.

-FER

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