By Skip Rigney

It has been hard to miss the brilliant, beautiful May sunshine over the last few days. The scarcity of clouds, combined with longer days and higher sun angles, have given us ample opportunity to appreciate the big, bright orb in the sky.

The annual cycle of changing day length and sun angle are the major drivers for our four seasons. That cycle has been recognized for millenia. Less obvious is the fact that these changes in sun angle and day length are related to the changing position of the Earth’s tilted axis relative to the sun from summer to winter and back again.

Yet three thousand years ago, Chinese and Indian astronomers postulated, not only that the Earth was round, but that its axis of rotation was tilted with respect to the sun.

They were able to estimate the approximate 23 degree angle of tilt surprisingly accurately by measuring and comparing the lengths of the shadows cast by a stick on the days of the summer solstice and the winter solstice.

Throughout the year, the Earth’s axis remains tilted toward the same stars in deep space. However, that means that, as the Earth orbits the sun, the angle of the Earth’s axis with respect to the sun flips between two extremes. During June at the summer solstice, the north pole is tilted most toward the sun. In December at the winter solstice, the north pole is tilted the most away from the sun. That corresponds to maximum and minimum daylight and sun angle in the Northern Hemisphere.

Currently, the Earth’s axis of rotation is tilted about 23.4 degrees from vertical. But, it’s been known for hundreds of years that the angle is not constant. In fact, the angle can vary a couple of degrees over tens of thousands of years.

In the 1920s Serbian scientist Milutin Milankovic demonstrated that, as the Earth’s tilt and also its distance from the sun varies over thousands of years, the amount of solar energy received by the Earth also changes significantly.

Those changes are responsible for many, though not all, long-term climate changes.

Changes in the Earth’s position affects the amount of solar energy we receive, but the amount of energy emitted by the sun varies also. The most studied variation is a roughly 11-year cycle of increasing and decreasing sunspot activity related to a 22-year cycle in the sun’s magnetic field.

Research continues on solar cycles that occur on even longer timescales. The extent to which these changes in the sun’s output affect the Earth’s climate is also a topic of ongoing research.

Last year, solar scientist Robert Leaman and his colleagues announced an intriguing finding. Each of the five times between 1960 and 2009 that the sunspot cycle abruptly reversed phases, oceanic conditions in the tropical Pacific flipped into the cool phase known as La Nina.

Leaman predicted that with a reversal in sunspot activity in mid-2020, La Nina would develop. That’s what happened, and, as is usually the case, La Nina caused changes in atmospheric circulation patterns that led to a very busy hurricane season. Leaman’s findings were published in February in the journal Earth and Space Science.

Those are just a few connections to ponder next time you’re enjoying the sunshine.