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Tilt of Earth and Solar System Planets
Many of us know that the Earth's axis doesn't stand upright as as we orbit around the Sun, but that it's tilted about 23.5 degrees out of vertical, and that's why we have our seasons. On June 21 -- on the June (or Northern) solstice -- the Earth's North Pole tilts most toward the Sun for the year. Then, six months later, at the December 22 Southern solstice, it's the Earth's South Pole that most tilts toward the Sun.
If, indeed, a picture is worth a thousand words, this animation of the tilting Earth orbiting the Sun must speak volumes. Keep in mind that showing proportion is next to impossible in many astronomical diagrams. Properly proportioned, the Sun's diameter would be some 109 times greater than the Earth's diameter, and the Earth's distance from the Sun would equal about 109 Sun diameters.
In general, the rotational axes of most of the solar system's planets are inclined, rather than perpendicular to their orbital planes. Surprisingly, the axial tilt of three planets nearly matches that of Earth: Mars: 25 degrees; Saturn: 27 degrees; and Neptune: 28 degrees. Uranus tilts over sideways, whereas Mercury has no appreciable tilt. Depending on your perspective, you could say that Venus tilts marginally or virtually upside-down. Here's a listing of the axial tilts of the planets.
Does the Sun's Rotational Axis Tilt?
The Sun's axis tilts almost 7.5 degrees out of perpendicular to Earth's orbital plane. (The orbital plane of Earth is commonly called the ecliptic.) Therefore, as we orbit the Sun, there's one day out of the year when the Sun's North Pole tips most toward Earth. This happens at the end of the first week in September. Six months later, at the end of the first week in March, it's the Sun's South Pole that tilts maximumly towards Earth.
There are also two days during the year when the Sun's North and South Poles, as viewed from Earth, don't tip toward or away from Earth. This happens at the end of the first week in in June, and six months later, at the end of the first week of December.
The Sun rotates once in about 25 Earth-days at its equator, but more like 27 Earth-days at 35 degrees from the equator. As seen from Earth, a long-lasting sunspot will return to the same place on the solar disk about 2 Earth-days later than the Sun's rotational period. That's because our orientation has changed, due to the Earth's revolution around the Sun. Traveling from east to west, sunspots go straight across the Sun's disk in June and December, but their paths dip somewhat in March and September, when the Sun's rotational axis tilts most toward Earth.*
All five naked-eye planets -- Mercury, Venus, Mars, Jupiter and Saturn -- are visible in June. With the exception of Venus, all these planets come out at nightfall. To see Venus, by far the brightest planet, you must be up in the wee hours before sunrise. You'll see Venus low in your eastern sky about one hour before sunrise.
As for the evening planets, Jupiter can't be missed. Much brighter than any star, Jupiter beams in the southern sky at early evening and sets in the southwest after midnight. Mercury, the innermost planet, might be your biggest challenge, though this month Mercury's showing is about as good as it gets. This retiring planet sets by the time it gets good and dark, so you must catch it at evening dusk, about 45 minutes to an hour after sunset. You'll want a clear view to the west, because it'll be low in the sky. Mid month is probably best, but anytime from June 5 to June 25 is worth a try. Look for Mercury below the Moon on June 27.
Ruddy Mars and golden Saturn might be challenging too, because they're not real bright. Saturn is brighter than Mars, however, and can be used to locate the red planet. Mars and Saturn are especially close at mid month, and make a great binocular target. These planets appear in the west at nightfall, and set late in the evening in early June. At the end of the month, however, Mars and Saturn don't stay up long after dark. Look now, because in July these planets fade into the evening twilight. The Moon and Mars easily occupy the same binocular field on the evening of June 28.
The rise/set times for the Sun, Moon and planets in your sky are available at this site, courtesy of Old Farmer's Almanac, or this site, courtesy of the US Naval Observatory. For a cool online planetarium that shows the positions of the planets and stars, check out Your Sky by John Walker.
copyright 2006 by Bruce McClure
May 2006 Feature * July 2006 Feature
Astronomy (Ninth Edition) by Robert H. Baker and Lawrence W. Fredrick The 2006 Astronomical Almanac, issued by the US Naval Obervatory and Her Majesty's Nautical Almanac Office