Advocates for ideas and draws conclusions based on the interpretation of facts and data.
The fall equinox is a significant event in any tipped planet's life. Seasons are caused by a planet's tilted axis. Those which orbit the sun standing nearly straight up and down with little to no axial tilt, like Mercury and Jupiter, don't experience seasonal changes. Earth's axis is inclined 23.5 degrees, which tilts the northern hemisphere toward the sun during summer. That shifts the sun's position to high in the sky, with longer daylight hours and more intense heating as consequences.
In winter, when the planet has moved to the opposite end of its orbit, the northern hemisphere faces away from the sun. The sun follows a low path in the sky, which shortens the number of daylight hours and ushers in colder weather.
The fall and spring equinoxes mark the mid-points between these day-length extremes, when day and night are nearly equal at 12 hours apiece. This year, autumn begins at 1:19 p.m. Central Time on Monday, Sept. 22. I enjoy the season for its smells and colors, cooler temperatures and earlier nights. Skywatchers can gaze at the moon or take in sumptuous views of the Milky Way and still get to bed by 10 or 11. As fall begins, the Big Dipper dips down into the northwestern sky and the Pleiades peep up in the east. It's a lovely time, really.
Mars's axis is tipped 25 degrees, almost the same as Earth's, so it also experiences seasons, along with Saturn (27 degrees), Uranus (98 degrees) and Neptune (28 degrees). The length of each planet's seasons varies with its distance from the sun. On Mars a season lasts roughly twice as long as it does on Earth. Imagine Saturn's leisurely summers which last about 7.5 Earth years. At Neptune, where a season spans 40 years, you'd only experience two in an average lifetime!
Uranus and Venus are oddballs. Uranus spins backwards because it's tipped more than 90 degrees from the vertical with its "butt" end up. Venus's axis is even more extreme, tilted a whopping 177.4 degrees -- rolled over so far that it rotates about 3 degrees from vertical. Compared to most of the other planets in the solar system it spins backwards. Why not just say that Venus is tipped 3 degrees and happens to rotate backwards? And what causes planets to spin backwards anyway?
The answer takes us all the way back to the solar system's birth when the planets formed within an enormous pancake-like rotating disk surrounding the newborn sun. As viewed from above Earth's north pole, which we use as the standard, that disk was spinning counterclockwise. The planets continue to revolve around the sun in the same direction to this day. Likewise, each individual planet shared in the same counterclockwise spin as it coalesced from the matter around it, so they all likely spun in the same direction in their youth.
But when we look at Venus today, it's rotating almost exactly clockwise. Something must have happened to tilt its axis to such an extreme -- 177 degrees -- to account for the contrary spin. Uranus also spins clockwise though less dramatically so. This leads us to the conclusion that their "flips" occurred sometime later, possibly from impacts by other newly-forming bodies or through gravitational interactions between the early planets as they migrated to their current locations in the solar system. Impacts may even have played a part in the tilted axes of Earth and other planets.
Like the season of fall, the solar system is incredibly dynamic. Things that seem to stay put for now were once in different places and may be again. There is an inherent dynamism to both living and non-living processes that ensure that change is the name of the game. With that in mind, we welcome the colors, the sinking sun and the return of the autumn stars.