Table of Epochs
| A. D. | Mercury. | Venus. | Earth. | Mars. |
|---|---|---|---|---|
| Period | 88.0 days. | 224.7 days. | 365.25 days. | 687.1 days. |
| 1900 | Feb. 18th. | Jan. 11th. | Sept. 23d. | April 28th. |
| 1901 | Feb. 5th. | April 5th. | Sept. 23d. | ... |
| 1902 | Jan. 23d. | June 29th. | Sept. 23d. | March 16th. |
| 1903 | April 8th. | Feb. 8th. | Sept. 23d. | ... |
| 1904 | March 25th. | May 3d. | Sept. 23d. | Feb. 1st. |
| 1905 | March 12th. | July 26th. | Sept. 23d. | Dec. 19th. |
| 1906 | Feb. 27th. | March 8th. | Sept. 23d. | ... |
| 1907 | Feb. 14th. | May 31st. | Sept. 23d. | Nov. 6th. |
| 1908 | Feb. 1st. | Jan. 11th. | Sept. 23d. | ... |
| 1909 | Jan. 18th. | April 4th. | Sept. 23d. | Sept. 23d. |
| 1910 | Jan. 5th. | June 28th. | Sept. 23d. | ... |
The first line of figures in this table shows the number of days that each of these planets requires to make a complete revolution about the sun, and it appears from these numbers that Mercury makes about four revolutions in its orbit per year, and therefore crosses the prime radius four times in each year, while the other planets are decidedly slower in their movements. The following lines of the table show for each year the date at which each planet first crossed the prime radius in that year; the dates of subsequent crossings in any year can be found by adding once, twice, or three times the period to the given date, and the table may be extended to later years, if need be, by continuously adding multiples of the period. In the case of Mars it appears that there is only about one year out of two in which this planet crosses the prime radius.
After the date at which the planet crosses the prime radius has been determined its position for any required date is found exactly as in the case of the earth, and the constellation in which the planet will appear from the earth is found as explained above in connection with Jupiter and Saturn.
The broken lines in the figure represent the construction for finding the places in the sky occupied by Mercury, Venus, and Mars on July 4, 1900. Let the student make a similar construction and find the positions of these planets at the present time. Look them up in the sky and see if they are where your work puts them.
31. Exercises.—The "evening star" is a term loosely applied to any planet which is visible in the western sky soon after sunset. It is easy to see that such a planet must be farther toward the east in the sky than is the sun, and in either [Fig. 16] or [Fig. 17] any planet which viewed from the position of the earth lies to the left of the sun and not more than 50° away from it will be an evening star. If to the right of the sun it is a morning star, and may be seen in the eastern sky shortly before sunrise.
What planet is the evening star now? Is there more than one evening star at a time? What is the morning star now?
Do Mercury, Venus, or Mars ever appear in opposition? What is the maximum angular distance from the sun at which Venus can ever be seen? Why is Mercury a more difficult planet to see than Venus? In what month of the year does Mars come nearest to the earth? Will it always be brighter in this month than in any other? Which of all the planets comes nearest to the earth?
The earth always comes to the same longitude on the same day of each year. Why is not this true of the other planets?
The student should remember that in one respect Figs. [16] and [17] are not altogether correct representations, since they show the orbits as all lying in the same plane. If this were strictly true, every planet would move, like the sun, always along the ecliptic; but in fact all of the orbits are tilted a little out of the plane of the ecliptic and every planet in its motion deviates a little from the ecliptic, first to one side then to the other; but not even Mars, which is the most erratic in this respect, ever gets more than eight degrees away from the ecliptic, and for the most part all of them are much closer to the ecliptic than this limit.