April 16, 1999
4. Western Christianity has a tolerably simple formula for determining the date of Easter. In this recipe, we wait for the first full moon after the vernal equinox - the following Sunday should be Easter Sunday. Since the equinox was March 21 and a full moon followed it on March 31, this first Sunday of April is Easter Sunday. The traditional link of the religious calendar to the astronomical calendar has been terribly strong and was a motivating force behind the calendar reforms of 1582.
6. This date is recorded as an anniversary of one of the first solar eclipses with a firmly established historical record, in the year 648 B.C. It's worth asking how meaningful that sort of anniversary is, since no calendar system remained in use for more than several centuries until modern times. Because solar eclipses are rare and the dates are calculable, these records actually make possible our translation of ancient calendar records to a system of dates that conforms to our usage.
8. The moon is at last quarter phase at 9:51 p.m., at which time it is still well below the horizon for mid-America. Check later on towards morning when the moon will be east of the star-studded region of Sagittarius. You can see clearly how very low the ecliptic path of the planets is at this time by noting Mars' very low position off towards the southwest.
9. How badly would you want to see Neptune and Uranus? At 4 a.m. tomorrow, Neptune will be a little over one degree south of the moon. Uranus will take a turn the following morning at about the same hour. Uranus is quite a bit brighter but its conjunction takes place earlier, when both planet and moon are barely above the southeastern horizon. You'll need binoculars or a small telescope to spot the greenish disk of Uranus. Be careful! Don't risk a surprise look at the rising sun in your binoculars!
14. The eastern horizon is thick with solar system objects this morning, with Saturn and Jupiter flanking the rising sun. Leading the parade by a bare hour or so are the moon and Mercury, about one degree apart. This will be tough to see, as Mercury will barely clear the horizon forty minutes or so ahead of the sun's glow.
15. The moon slips between the Earth and sun at 11:22 p.m., skating several degrees south of the sun's position. Since the moon never steps in front of the sun today, there is no possibility of eclipse, but we do get to start a new lunar month with the new moon.
18. West is definitely the direction to turn this evening, as the moon and Taurus' bright giant star, Aldebaran, stage a close approach. At around 9 p.m., Aldebaran and the moon are about as close as they'll get (less than a degree apart). Venus poses some seriously bright competition for your attention to the right of the pair.
21. The moon has moved out of the frame, but Venus and Aldebaran still make a lovely pair in the western evening sky. Aldebaran is one of the 20 brightest stars in the sky, but it can't compete with Venus' brilliance. The moon will reach first quarter phase tomorrow at 2:01 p.m.
24. This evening features a nice conjunction of the waxing moon and Regulus, the bright star of the constellation Leo. Mars is at opposition today, which places it exactly opposite the sun's position and at its closest distance from Earth, about 60 million miles away. Off to the west, Venus is exiting the evening stage as it sets with the stars of another zodiac constellation, Taurus.
27. As the sun crosses from horizon to horizon today, Saturn is very near its position on the sky. You'll have to take our word that Saturn lies beyond the sun; it's not at all safe to try to look for it in the daytime. Jupiter and Mercury are further west of the sun, so they may be safely seen in the half hour before sunrise.
28. The career of Sir Isaac Newton provides a wonderful illustration of how personality and friendships play a role in the development of science. Newton was proud, shy and didn't much care for criticism; he was willing to work forever in isolation rather than risk an error in public. Sir Edmond Halley, though, was more of a mover. It was due to Halley's encouragement (more like nagging) that Newton's great work Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) was published on this date in 1686.
29. The moon will be full tomorrow at 9:55 a.m. This evening, it joins another solar system object near opposition to the sun, the planet Mars. Mars should be brighter than any of the stars around but nowhere near the brightness of the full moon.
30. Carl Friedrich Gauss was born on this date in 1777. Not exactly a household name, you say? It probably should be; Gauss is arguably one of the greatest mathematicians in history. His contributions to statistics, algebra and number theory are huge, while physicists use his mathematical expressions for the spatial distribution of force fields every day. Gauss' greatest triumph in the arena of astronomy was a method of calculating solar system orbits after just three observations. He was able to compute the orbit of the newly discovered minor planet, Ceres, in 1801, so precisely that Ceres was recovered neatly once it re-emerged from behind the sun.