site sides of the ecliptic, it follows that the earth must pass the line of the nodes of the interior planets, respectively, in opposite months of the year. These months are called the node months of the planet, and are the months in which all its transits must occur.

114. In making transits across the sun's disk, the planets seem to pass from east to west, and to ascend or descend, as respects the ecliptic, according as the planet is at the ascending or descending node.

This variation in the direction of the planets, during different transits, is well represented in the next cut.

115. The node months of Mercury are May and November.

All the transits of Mercury ever noticed have occurred in one or the other of these months, and for the reason already assigned. The first ever observed took place November 6, 1631; since which time there have been 29 others by the same planetin all 30-8 in May, and 22 in November.

116. The last transit of Mercury occurred November 9, 1848; and the next will take place November 11, 1861. Besides this, there will be five more during the present centurytwo in May, and three in November.

The accompanying cut is a delineation of all the transits of Mercury from 1802 to the close of the present century. The dark line running east and west across the sun's center represents the plane of the ecliptic, and the dotted lines the apparent paths

of Mercury in the several transits. The planet is shown at its nearest point to the sun's center. His path in the last transit and in the next will easily be found.

2. The last transit of Mercury was observed in this country by Professor Mitchel, at the Cincinnati Observatory, and by many others both in America and in Europe.

113. What are the node months? (Explain by diagram.)

114. In what direction do planets cross the sun in transits, and why? 115. Which are the node months of Mercury? 116. When did the last transit of Mercury occur? take place? (What represented in the cut? Describe. shown? What said of last transit of Mercury?)

When will the next
Where is the planet

The writer had made all necessary preparation for observing the phenomenon at his residence, near Oswego, New York; but, unfortunately, his sky was overhung with clouds, which hid the sun from his view, and disappointed all his hopes.

117. The node months of Venus are December and June. The line of her nodes lies in Gemini (II) and Sagittarius (); and as the earth always passes those points in the months named, it follows that all transits of Venus must occur in those months for ages to come.

This proposition will be well understood by consulting the cut on page 61; for as the ine of Venus's nodes is only one sign ahead of that of Mercury, the earth will reach hat point in the ecliptic in one month after she passes the line of Mercury's nodes; so hat if his transits occur in May and November, hers should occur in June and Decem Jer, as is always the case.

118. The last transit of Venus occurred June 3, 1769; and the next will take place December 8, 1874.

1. Only three transits of Venus have as yet been observed-namely, December 4, 1639; June 5, 1761; and June 3, 1769. It is said that Rittenhouse was so interested in viewing that of 1769, that he actually fainted. In defining the term transit, Dr. Webster says: I witnessed the transit of Venus over the sun's disk, June 3, 1769." (See "Unabridged" Dictionary.) The next four will occur December 8, 1874; December 5, 1882; June 7, 2004; and June 5, 2012.

2. The first transit ever witnessed was that of December 4, 1639. The observer was, a young man named Horrox. living in an obscure village near Liverpool, England. The cable of Kepler, constructed upon the observations of Tycho Brahe, indicated a transit of Venus in 1631, but none was observed. Horrox, without much assistance from books and instruments, set himself to inquire into the error of the tables, and found that such a phenomenon might be expected to happen in 1639. He repeated his calculations during this interval with all the carefulness and enthusiasm of a scholar ambitious of being the first to predict and observe a celestial phenomenon which, from the creation of the world, had never been witnessed. Confident of the result, he communicated his expected triumph to a confidential friend residing in Manchester, and desired him to watch for the event, and to take observations. So anxious was Horrox not to fail of witnessing it himself, that he commenced his observations the day before it was expected, and resumed them at the rising of the sun on the morrow. But the very hour when his calculations led him to expect the visible appearance of Venus on the sun's disk, was also the appointed hour for the public worship of God on the Sabbath. The delay of a few minutes might deprive him forever of an opportunity of observing the transit. If its very commencement were not noticed, clouds might intervene, and conceal it until the sun should set; and nearly a century and a half would elapse before another opportunity would occur. He had been waiting for the event with the most ardent anticipation for eight years, and the result promised much benefit to the science. Notwithstanding all this, Horrox twice suspended his observations, and twice repaired to the house of God, the great Author of the bright works he delighted to contemplate. When his duty was thus performed, and he had returned to his chamber the second time, his love of science was gratified with full success, and he saw what no mortal eye had observed before. If any thing can add interest to this incident, it is the modesty with which the young astronomer apologizes to the world for suspending his observations at all. "I observed it," says he, "from sunrise till nine o'clock, again a little before ten, and lastly at noon, and from one to two o'clock; the rest of the day being devoted to higher duties, which might not be neglected for these pastimes."

3. The transit of 1769 was observed with intense interest by astronomers in both hemispheres. To secure the advantages of observations at different points, Capt. Cook was

117. Node months of Venus? Where line of nodes? Why June and December her node months? (Why only one month after those of Mercury?) 118. When last transit of Venus? Next? (How many have been observed? What said of Rittenhouse? Webster? When next four transits of Venus? When first transit noticed? What said of it? That of 1769? Cook-use of observations?)

sent to the Pacific in the bark "Endeavor," where he perished subsequently by the hands of savages at one of the Sandwich islands. Observations upon these transits furnish data for important astronomical calculations.

119. In consequence of the earth's annual revolution around the sun, he appears to travel eastward, through all the signs of the zodiac, every 365 days. It is this eastward motion of the sun that causes the stars to rise and set earlier and earlier every night.

SUN'S APPARENT MOTION AROUND THE ECLIPTIO.

Let a person walk around a tree, for instance, at a short distance from it, and it will appear to sweep around the horizon in an opposite direction. So as the earth revolves annually about the sun, the sun appears to traverse the circle of the heavens in the opposite direction. Suppose the earth is at A on the 20th of March; the sun will appear to be at B in the opposite side of the ecliptic. As the earth moves on in her orbit from A to C, the sun will appear to move from B to D; and will seem thus to traverse the whole circle of the heavens every 365 days, or as often as the earth revolves around him. The time of the sun's apparent entrance into the different constellations, as he journeys eastward, is usually laid down in almanacs. Thus: "Sun enters (Artes) 20th of March, &c. ;" at which time the earth would enter the sign (Aquarius), and the sun would seem to enter the opposite sign Aries.

119. What said of sun's apparent motion? Cause? Time of revolution? Effect upon the stars? (Illustration from tree? By diagram.) What is meant by the sun's entering Aries? When? Where earth ther

CHAPTER II.

PRIMARY PLANETS

CONTINUED.

120. BESIDES the revolution around the sun, the planets all revolve rapidly about their respective axes, as they perform their celestial journeys. This is called their diurnal revolution.

The evidences of the earth's revolution have already been considered on pages 13 and 14. That most of the other planets revolve has been ascertained by carefully observing the motions of spots, as they seemed to pass periodically over their disks.

121. The axis of the earth is inclined to the plane of the ecliptic 23° 28'. It is always parallel to itself—that is, it always inclines the same way, and to the same

amount.

INCLINATION OF THE EARTH'S AXIS TO THE PLANE OF THE ECLIPTIC.

1. The inclination of the earth's axis, and its parallelism to itself, are exhibited in the above cut, as also in the cuts, pages 50, 51, and 64, to which the student will do well to turn.

2. The author is aware that the poles of the earth have a slow motion around the pole of the ecliptic, requiring 25,000 years for a single revolution, but prefers to consider this point hereafter, in connection with the precession of the equinoxes.

122. The axes of all the planets are inclined more or less to the planes of their respective orbits. This inclination, so far as known, is as follows:

120. What revolution have the planets besides around the sun? What called? (What proof of the earth's revolution? Of the other planets?) 121. What said of the axis of the earth? Of the stability of its inclination? (Is there no variation?)

122. Are the axes of the other planets inclined? To what extent, respectively? (Substance of note 1? Illustrate by diagram. Note 2?)

1. The student will bear in mind that the above inclination is not to the ecliptic, or plane of the earth's orbit, but to the plane of the orbits of the several planets respectively. Take the case of Venus, for instance:

PLANE OF VENUS' ORBIT

PLANE OF THE ECLIPTIC

The orbit of Venus departs from the ecliptic 310, as stated at 108, while her axis is inclined to the plane of her orbit 750, as shown in the above figures. This distinction should be kept definitely in view by the student.

2. The inclination of the axes of the several planets, each to the plane of its own orbit, is represented in the following cut:

INCLINATION OF THE AXES OF THE SEVERAL PLANETS TO THE PLANES OF THEIR ORBITS.

123. The inclination of the earth's axis to the plane of the ecliptic causes the equinoctial to depart 23° 28′ from the ecliptic. This angle made by the equinoctial and the liptic is called the Obliquity of the Ecliptic.

Let the line A A represent the axis of the earth, and BB the poles or axis of the ecliptic. Now if the line A A inclines toward the plane of the ecliptic, or, in other words, departs from the line B B to the amount of 230 28', it is obvious that the plane of the

123. What effect has the inclination of the earth's axis upon the equinoctial? What is the obliquity of the ecliptic? (Illustrate by diagram.)