Earth reaches its farpoint on the Fourth of July. This is aphelion, when we are 94 million miles from old Sol. This three-percent increase in our distance since January yields a seven-percent reduction in solar brightness. It doesn’t feel like it. That’s because the Sun’s greater elevation is of more importance to sunray intensity.
But having sunlight seven percent weaker does partially counteract our Northern Hemisphere being maximally tipped sunward. It makes our summers and winters more moderate. In 10,000 years, when we’ll be tipped toward the Sun just as the Sun is nearest, seasons will be much fiercer. Right now we have the best of all worlds.
You’d think this situation would make the Southern Hemisphere very unpleasant. It’s currently their winter because they’re tilted away from the sun, and the Sun is most distant. Shouldn’t their winters be ultra-bitter? Turns out, the Southern Hemisphere’s much greater ocean area has such a moderating influence that it balances out their immoderate sunray situation. They ended up with seasons similar to ours.
Another consequence of the Sun now being so far away was spelled out by Johann Kepler in the 17th century. He said that whenever a planet is closest to the Sun, the stronger gravity whips it around at a faster speed. In other words, Earth moves fastest in early January and slowest right now. And not by a little bit: Our planet travels around 67,000 miles per hour on New Year’s Day, and only 65,000 miles per hour right now. Does this variation produce its own consequences?
You bet. If our orbital speed keeps changing but our daily spin is constant, then this affects the Sun’s position in our sky. We created a timekeeping system, a “day,” based on the average interval between the Sun reaching the meridian and its next meridian transit. That average interval is 24 hours. But in January, we’re circling the Sun so quickly that a single axial spin is insufficient to have us again face the Sun the next day. We need to spin for an extra few minutes to face the noon Sun directly again. This throws sundials out of kilter. It makes each day’s highest Sun happen up to 16 minutes later than clock-time noon. And since lunar phases also depend on the Sun’s position, it makes the Full Moon-to-Full Moon interval vary greatly during the year. Isaac Newton said that just thinking about all this gave him a headache.
So we’ll just keep it simple. If you feel sluggish on Independence Day, it may not be due to too many hot dogs or too much humidity. That day, the entire world is at its most lethargic.
Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at HudsonValleyAlmanacWeekly.com.