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Astronomical delights

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Witness Halley's Comet and a host of other stellar performers with build-it-yourself starfinder hardware and software.

January 1986 will host a once-in-a-lifetime event for most people - the return of Halley's Comet after a 76-year absence. If you want to get in on the fun and join the ranks of amateur astronomers, build a starfinder out of cardboard that, when used with a Model I/I11/4/1000 program called Sidereal, lets you locate celestial objects such as stars, nebulas, planets, and comets with the naked eye.

Star search

All professional and serious amateur astronomers locate and observe celestial objects with a telescope mounted on an axis parallel to the axis of the earth, a setup known as an equatorial mount system. The setting circles device you'll build is a miniature version of an equatorial mount, with a viewfinder instead of a telescope (see Fig. 3h for a finished version.

You'll locate heavenly bodies with the setting circles and a program called Sidereal, which measures sidereal time (see the Program Listing). Sidereal time is a measurement of time relative to the stars (by contrast, we measure "normal" time with respect to the sun).

Knowing the correct sidereal time is crucial in locating heavenly objects, especially if you're unfamiliar with the constellations. Until the advent of the small computer, you could derive sidereal time only from tables or expensive sidereal clocks. Now all you need is about 1 K of RAM.

Assembling the setting circles

To assemble the setting circles, first make photocopies of Figs. 1 and 2 (pp. 62 and 66). It's important to make the photocopies as close as possible in quality to the originals as you'll use the setting circles outdoors under poor lighting conditions.

Fig 1
Figure 1. Three elements of the setting circles device.

Mount the photocopies on good-equality cardboard, about 1/16 of an inch thick. This thickness should be just thin enough to allow folding with slight difficulty, but thick enough to be rigid when you assemble the device. Rigidity is important because if you use cardboard that's too thin, it will absorb moisture in the cool night air and become flimsy, losing its accuracy.

You can mount the circles on cardboard by applying a spray-type adhesive to both surfaces and carefully joining them. You can also dry-mount them (or have a photo studio do so). I don't recommend using liquid cement or glue as they tend to wrinkle the photocopies.

After you mount the copies, carefully cut out all five elements along the thick black lines. Then, along the dotted lines marked "Fold," cut the cardboard with a hobby knife to a depth of about one-third the thickness of the cardboard to make folding easier.

Fig 3
Figure 3 a-h. Assembling the setting circles device.

Fold the base as shown in Fig. 3a. What you do now depends on where you live. For example, if you live in Minneapolis, MN, your latitude is 45 degrees north. You would score the latitude scale with a knife at the point marked "45" to the focal point of the converging lines (see indicating arrows in Fig. 3b). Then you would fold along this link. If you lived south of the equator, say at -45 degrees south latitude, follow the same procedure.

Fold the base as shown in Fig. 3c, carefully aligning the latitude segment with the guide marks on the outer face of the semicircle marked "Sidereal Time." Staple or glue in place. Perforate with a nail at the point marked "Pivot" as shown in Fig. 3d

Assemble the right ascension arm as shown in Fig. 3e, maMng sure that the plane of the small pivot segment lies at an exact right angle to the large right ascension/declination segment. Now perforate at the point marked "Pivot."

Of the two circles, find the one with the star named Polaris near the centre and on the basck write "North Celestial Pole"; on the other circle, write "South Celestial Pole." If you live north of the equator, perforate the wheel marked "South Celestial Pole" and mount it on the base as shown in Fig. 3f. If you live south of the equator, follow the same procedure, but mount the wheel marked "North Celestial Pole."

You can use a small machine bolt as an axle for the polar wheel and the right ascension arm. Tighten the bolt enough so that friction holds the right ascension arm and polar wheel at any position without slipping. Fold and staple the viewfinder tube as shown in Fig. 3g and secure it temporarily to the right ascension arm as shown in Fig. 3h.

Introducing Sidereal Time

Now turn on your computer and type in Sidereal, making sure you have all the large constants correct. Run the program and it will prompt you to enter your longitude with respect to Greenwich, England, the locale that serves as the basis for standard time worldwide.

Again assuming you live in Minneapolis, MN, your longitude is approximately 93.2 degrees west of Greenwich. Therefore, you would answer the longitude prompt with a negative number (-93.2). If y;ou lived east of Greenwich, say 40 degrees east, you would answer the prompt with a positive number (40.0).

Sidereal then prompts you for the current date and the Universal time. You can find Universal time by listening to any international short-wave broadcast station. You can also calculate itd by accounting for how many hours off Greenwich Mean Time you are.

For example, if you live in Minneapolis, you are in the Central time zone, six hours behind Greenwich time. If it is 22:32 standard time in Minneapolis (on a 24-hour clock), Universal time is 22:32 plus six hours, or 28:32. Subtract 24 hours if necessary so that the result falls within the normal range of zero to 24 - 28:32 Universal time translates to 4:32 Universal time.

Table. Selected celestial objects.
Celestial object and descriptionRight ascension (A)Declination (B)
Sirius. The brightest star in the sky.6h 42.9m-16° 39'
Great nebula in Orion. Visible to the naked eye as a hazy object.5h 32.9m-5° 25'
Pleiades. Spectacular cluster of seven stars. Many more stars are visible with optical aid.3h 44m+ 5° 25'
Mira (Wonderful Star). Varies in magnitude from 3.4 to 9.2 over a period of 332 days. If you don't see it, It may be below naked-eye visibility.2h 18.8m-3° 12'
Great Andromeda nebula. External galaxy. The most distant object seen with the unaided eye. It is 1.5 million light years distant. It appears as a slightly elongated hazy object.Oh 40m+41° 00'
Center of our galaxy. An interesting region rich with stars along the Milky Way.17h 42m-38° 30'
Antares. A star with a diameter 300 times that of the sun. Orange in color.16h 26.3m-26° 19'
h = hours m = minutes ° = degrees ' = minutes

The following list indicates the hours behind Greenwich Mean Time of each U.S. time zone:

Say you answered Sidereal's prompts for June 1, 1985, at 1:35 Universal time. Sidereal gives you the following results:

UNIVERSAL TIME = 1.35
JULIAN DATE = 2446216.5
LOCAL APPARENT SIDEREAL TIME = 11:59
GREENWICH APPARENT SIDEREAL TIME = 18:13
NEW TIME?

The local apparent sidereal time is the only data you need to work the setting circles (if you are currently using Daylight Savings rune, ignore that fact and calculate Universal time using standard time). On June 1, 1985, 1:35 Universal time in Minneapolis is 11:59 sidereal time. (The Greenwich apparent sidereal time, 18:13, is the sidereal hour at Greenwich at 1:35 Universal time. The Julian Date is important in astronomical computations; it is a count of days from noon on Jan. 1, 4713 B.C.)

You can enter a new time i ncrement, Universal time, at the "New time?" prompt without having to enter all the other data. After you do so, the program returns new results.

Working with the setting circles

Now you're ready for the stars. Choose an outdoor observation site with a clear, unobstructed view to the north (to the south if you live in the southern hemisphere). The best platform for the setting circles is a camera tripod, but you can also use a pier or a 2- by 4-inch board with one end buried in the ground. The top should not be so high as to keep children from using it. You can use a table or any level surface, but it's not the best solution because it limits free access to some sides of the device.

Fig 4
Figure 4 a-f. Aligning and using the setting circles.

On a cloudless night when you have a clear view to the north (or south if you live south of the equator), compute the proper sidereal time for your viewing site. Mark with a paper clip your correct sidereal time on the outer edge of the wheel that is not mounted. Now hold the wheel as shown in Fig. 4a.

If you're in the Northern Hemisphere, look north with the paper clip pointing straight up. Find the pole star, Polaris. The star field printed on the wheel should coincide with what appears in the heavens. The pole star is easy to identify as it is the last star in the handle of the little Dipper (ursa Minor). If the Big Dipper (Ursa Major) is in the sky, you can also use it to orient yourself. The outer two stars of the Big Dipper align with Polaris.

In the Southern Hemisphere, you'll find it more difficult to orient the device to the south celestial pole because no distinguishable pole star exists as in the north. Looking south, align the viewfinder with the area of sky that corresponds to the exact centre of the unmounted wheel (-90 degrees). (Incidentally, viewing Halley's Comet will be better the farther south you happen to be.)

Set the sidereal time wheel to zero hours (C in Fig. 4e). Now set the right ascension segment to zero hours (A in Fig. 4c). Set the declination pointer to +90 degrees and secure with a paper clip (B in Fig. 4d).

Put the device on a level surface and, while looking through the viewfinder (see Fig. 4b), locate Polaris (north) or the area of centre sky (south). Without moving the device, secure it with tape or tacks if you're using a wood surface, or a machine bolt if you're using a camera tripod head. The setting circles are now aligned and ready to use.

Searching the Heavens

Astronomers pinpoint celestial objects using a system of coordinates similar to the terrestrial longitude and latitude system. Right ascension is expressed in hours, minutes, and seconds of time. The markings along the outer edge of the star field wheel correspond to right ascension. Declination, the angular distance of a celestial body north or south of the celestial equator, corresponds to latitude on earth. The declination marker is used for this purpose.

To get you started, I have prepared a list of interesting celestial objects that you can see from the northern and southern hemispheres (see the Table). Your geographical location, time of year, and sidereal time will determine which of these objects you'll be able to see.

Compute the correct sidereal time and choose an object from the Table whose right ascension is close to your sidereal

time I suggest a star rather than a nebula for your first try. Set the right acension wheel (A) to the object's right ascension (see Fig. 4c). Now set the declination pointer (B) to the object's declination (see Fig. 4d).

Compute the exact local sidereal time and move the star field wheel so that the hour markings align with the sidereal time pointer (C in Fig. 4e). Make sure that when you move the star field wheel, the right ascension arm moves with it. Look through the viewfinder as shown in Fig. 4f and you should be able to find your chosen object within the enclosed area of the viewfinder.

Now try some of the other objects within range by repeating the above process. If the object is out of viewing range at the tme you are observing, the viewfinder will point below the horizon. Some of the objects, such as nebulas, are difficult for the untrained eye to find. If you live in the city, these objects may be impossible to see because of "light population."

If your computer isn't conveniently located near your viewing site, you can transfer the sidereal time to your wristwatch. First compute the present sidereal time and set your watch to that hour. The sidereal day is shorter than the solar day by about four minutes; therefore, every six hours set your watch back one minute. Keep in mind that we are using military (24-hour) time and you should remember this if you're using a conventional watch.

Viewing Planets and Comets

Because the stars and distant nebula are relatively stationary with respect to the astronomical coordinate system, we can safely list their present coordinates and the information will be accurate enough for reference for many years to come. This is not true, however, with planets, as they are constantly moving along a line called the ecliptic. Comets move in even more irregular orbits with respect to our viewpoint. Therefore, it is not possible to publish planetary or cometary positions as we do star.

Magazines such as Sky and Telescope and Astronomy publish planetary and cometary positions on a monthly basis. They also publish stellar maps and books that orient amateur astronomers. Refer to them and use your program and setting circles device to locate solar system bodies. Remember that best viewing for Halley's Comet will probably be in the springtime - and you want to be ready for it!

Program listing sidereal
2 REM 	 SIDEREAL 	
5 DEFDBL A-Z
10 CLS:PRINT"SIDEREAL":PRINT
20 PRINT"ENTER YOUR LONGITUDE,(+) IF EAST AND (-) IF WEST OF GREENWICH"
30 INPUT"(USE A DECIMAL POINT TO SEPARATE DEGREES AND MINUTES)";LG
40 ZZ=FIX(LG):XX=LG-ZZ:XX=XX*.016666666#*100:LG=ZZ+XX
50 INPUT"ENTER YEAR (RANGE = 1950 TO 2000)";K
60 INPUT"ENTER MONTH (1 TO 12)";M
70 INPUT"ENTER DAY OF MONTH";I
80 PRINT"ENTER UNIVERSAL OR GREENWICH MEAN TIME IN HOURS AND MINUTES"
90 INPUT"(USE A DECIMAL POINT TO SEPARATE HOURS AND MINUTES)";UTHM
100 CLS
110 PRINT"UNIVERSAL TIME =";UTHM
120 UD=(1.6666666671*(UTHM-INT(UTHM))+INT(UTHM))
130 JD=367*K-INT(7*(K+INT((M+9)/12))/4)+INT(275*M/9)+I+1721013.5#+UD/24-.5*SGN(100*K+M-190002.5#)+.5
140 J0=367*K-INT(7*(K+INT((M+9)/12))/4)+INT(275*M/9)+I+1721013.5#-.5*SGN(100*K+M-190002.5#)+.5
150 T0=(J0-2451545#)/36525#
160 T=(JD-2451545#)/36525#
170 GMST=6.697374560000002#+2400.051336#*T0+2.58622E-05*T0^2+1.002737909#*UD
180 IF GMST>=0 THEN GOTO 210
190 GMST=GMST+24
200 IF GMST<0 THEN GOTO 190
210 LNODE=125.04452#-1934.13626#*T+.002071*T^2
220 LNODE=LNODE/57.29577951000001#
230 E=-.00029*SIN(LNODE)
240 GAST=GMST+E
250 LAST=GAST+LG/15
260 IF LAST<0 THEN LAST=LAST+24
270 PRINT"JULIAN DATE";J0
280 HOUR=INT(LAST):DE=LAST-HOUR:MM=INT(DE*100):MM=MM/100:MM=MM*60:MM=INT(MM+.5)
290 PRINT"LOCAL APPARENT SIDEREAL TIME =";HOUR;":";MM
300 HUR=INT(GAST):DE=GAST-HUR:MM=INT(DE*100):MM=MM/100:MM=MM*60:MM=INT(MM+.5)
310 PRINT"GREENWICH APPARENT SIDEREAL TIME =";HUR;":"MM
320 PRINT:PRINT"	
330 INPUT"NEW TIME";UTHM:GOTO 110
340 END

Michael F. O'Reilly