699. Public observatories in this country? Largest telescope?

Table? Priva

observatories-names? Telescopes-by whom mostly made? What other table?

CHAPTER XX.

PROBLEMS AND TABLES.

PROBLEM I.

TO CONVERT DEGREES, ETC., INTO TIME.

RULE L-Divide the degrees by 15, for hours; and multiply the emainder, if any, by 4, for minutes.

2. Divide the odd minutes and seconds in the same manner by 15 for minutes, seconds, &c., and multiply each remainder by 4, for the next lower denomination.

EXAMPLE 1.-Convert 32° 34′ 45′′ into time.

EXAMPLE 2.-If it is 12 o'clock at this place, what is the time 20° cast of us?

Thus fifteen in 20°, once, and five over; the once is 1 hour, and the 5 multiplied by 4, gives 20 minutes; the time is then 1 hour and 20 minutes past 12.

EXAMPLE 3.—The longtitude of Hartford is 72° 50' west of Greenwich; what time is it at Greenwich when it is 12 o'clock at Hartford? Ans. 4h. 51m. 20s.

EXAMPLE 4.-When it is 12 o'clock at Greenwich, what is the time at Hartford? Ans. 7h. 8m. 40s.

PROBLEM II.

TO CONVERT TIME INTO DEGREES, ETC.

RULE.-Multiply the hours by 15, and to the product add onefourth of the minutes, seconds, &c., observing that every minute of time makes 1°, and every second of time '.

EXAMPLE 1.-In 2 hours, 10 minutes, and 19 seconds; how many degrees?

Thus ;

2h. 10m. 19.

Ex. 2. When it is 12 o'clock at Hartford, it is 4 hours, 51 minutes, and 20 seconds past noon at Greenwich; hòw many degrees is Hartford west of Greenwich?

Thus 15 times 4 is 60-added to of 51, is 72° 45′′, and this increased by of 20, is 72° 50′. Ans.

Ex. 3.-A Liverpool packet, after sailing several days from New York, finds the time by the Sun 2 hours and 40 minutes later than by the ship's chronometer: how far has the ship progressed on her way?

Ex. 4.-A vessel leaves Boston, and having been tossed about in foul weather for some days, finds, that when it is 12 o'clock by the Sun, it is only 11 o'clock and 50 minutes by the watch; is the vessel east or west of Boston; and how many degrees?

Ex. 5.-The moment of greatest darkness, during the annular eclipse of 1831, took place at New Haven, 10 minutes after 1 o'clock. A gentleman reports that it happened precisely at 1, where he observed it; and another that it was 5 minutes after 1 where he saw it; Query. How far east or west were these gentlemen from each other, and how many degrees from New Haven ?

PROBLEM III.

TO FIND WHAT STARS ARE ON THE MERIDIAN AT NINE O'CLOCK IN

THE EVENING OF ANY GIVEN DAY.

RULE.-Look for the given day of the month, at the bottom of the maps, and all the stars having the same degree of right ascension will be on the meridian at that time.

EXAMPLE 1.-What stars will be on the meridian at 9 o'clock, the 19th of January?

Solution.-On Map III. I find that the principal stars standing over against the 19th of January, are Rigel and Capella.

Ex. 2.—What stars are on the meridian the 20th of Decem.. ber? Ans. Menkar and Algol.

PROBLEM IV.

ANY STAR BEING GIVEN, TO FIND WHEN IT CULMINATES.

RULE. Find the star's right ascension in the table, or by the map (on the equinoctial), and the day of the month at the top or bottom of the map will be the day on which it culminates at 9 o'clock.

EXAMPLE 1.-At what time is the bright star Sirius on the meridian?

Solution.-I find by the table, and by the map, that the right ascension of Sirius is 6 hours and about 38 minutes; and the time corresponding to this, at the bottom of the map is the 11th of February.

Ex. 2.-At what time is Alpheratz, in the head of Andromeda, on the meridian? Ans. The 9th of November.

PROBLEM V.

THE RIGHT ASCENSION AND DECLINATION OF A PLANET BEING GIVEN, TO FIND ITS PLACE ON THE MAP.

RULE. Find the right ascension and declination of the planet on the map, and that will be its place for the given day.

EXAMPLE 1.-Venus's right ascension on the 1st of January, 1833, was 21 hours, 30 minutes, and her declination 164° south; required her situation on the map?

Solution. On the right hand of the Plate II. I count off 164° from the equinoctial, on the marginal scale south, and from that point, 30 minutes to the left or just half the distance between the XXI. and XXII. meridian of right ascension, and find that Venus, that day, is within two degrees of Delta Capricorni, near the constellation Aquarius, in the zodiac.

Ex. 2.-Mars' right ascension on the 13th of March, 1833, is 5 hours, 1 minute, and his declination 244° north; required his situation on the map?

Solution. I find the fifth hour line or meridian of right ascension on Plate III.., and counting upward from the equinoctial 244°, I find that Mars is between the horns of Taurus, and about 5° S. W. of Beta Auriga.

Ex. 3.-Required the position of Jupiter and Saturn on the 13th of February and the 25th of May?

PROBLEM VI.

TO FIND AT WHAT MOMENT ANY STAR WILL PASS THE MERIDIAN ON A GIVEN DAY.

RULE.-Subtract the right ascension of the Sun from the star's right ascension, found in the tables: observing to add 24 hours to the star's right ascension, if less than the Sun's, and the difference will show how many hours the star culminates after the Sun.