Why Are Seasons Reversed in the Southern Hemisphere?



Why Are Seasons Reversed in the Southern Hemisphere?

Have you ever talked on the phone with a friend who lives in the opposite hemisphere? It can be an eye-opening experience, particularly when they start complaining about the weather. While they’re experiencing icy winters and cold, bitter winds, you’re sweating it out in your t-shirt and shorts, trying to beat the summer heat.

But why do the northern and southern hemispheres have opposite seasons? To answer that, we should first take a step back and look at what causes seasonal weather shifts in the first place.

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A Primer on Seasons

Credit: BrianAJackson / iStockPhoto

We’d explain the concept of seasons, but why not let National Geographic do it instead?

A season is a period of the year that is distinguished by special climate conditions. The four seasons — spring, summer, fall, and winter — follow one another regularly. Each has its own light, temperature, and weather patterns that repeat yearly.”

Of course, the classic four season framework applies only to regions at mid-latitudes between the equator and the poles. Seasons are largely dependent on the region’s location relative to the equator, and as you travel closer to or further from the equator, this pattern begins to shift.

Closer to the poles, temperatures are generally colder with fewer hours of daylight. (In Barrow, Alaska, it’s consistently dark throughout most of the winter — close to three months!) But nearer to the equator, it’s warm for most of the year, and daylight cycles stay consistent.

In other words, seasonal shifts are determined by two things:

  1. The region’s location on the globe
  2. The axis of the earth relative to the sun.

That first point is a factor in explaining how extreme seasonal weather shifts can be. But when explaining why seasons are opposite across northern and southern hemispheres, the axis makes all the difference.

The Axis of the Earth Is Key

Credit: sundown001 / iStockPhoto

Our earth has a tilted axis relative to the position of the sun, which is why seasons are opposite across hemispheres.

The Extremes: Summer and Winter

Credit: SUNG YOON JO / iStockPhoto

When Earth’s axis is tilted such that the northern hemisphere leans towards the sun, those regions receive more solar energy, and thus, feel hotter. At the same time, the southern hemisphere receives very little solar energy, producing cold weather. Six months later, the opposite occurs—the other hemisphere tilts towards the sun, and the cycle continues.

The Middle Ground: Autumn and Spring

Credit: SrdjanPav / iStockPhoto

So, winter and summer are opposite. But what about autumn and spring?

These are even easier to understand. Since Earth’s axis produces a tilt that creates opposite seasons across the equator, there’s a sort of “middle ground” that occurs as Earth spins towards its summer/winter extremes. This middle ground is, essentially, the autumn and spring seasons.

Seasons Aren’t so Different

Credit: LeManna / iStockPhoto

During these mild seasons, both hemispheres receive the same amount of solar radiation, producing similar weather conditions across the north and south. The key difference comes from each region’s starting point.

When a region moves into autumn, it’s moving from a period of high solar energy (summer) into a lower period. And conversely, regions moving from winter to spring slowly gain solar energy. In this way, autumn and spring are functionally the same thing. The only difference is where each region begins.

We May Finally Know Where Vicious ‘Black Widow’ Pulsars Come From



We May Finally Know Where Vicious ‘Black Widow’ Pulsars Come From

Pulsar in Binary System

An illustration shows a pulsar in a binary system.
(Image: © ESA)

Vicious, fast-blinking “black widow” and “redback” pulsars dot the night sky. These violent stars blast their smaller stellar partners to bits as they whip them around in tight binary orbits, cannibalizing the smaller partners in the process. And, in a new paper, scientists have revealed the origin story behind these hungry stars.

It’s no coincidence that astronomers named these systems — places in space where a tiny, heavy, fast-spinning neutron star is energizing itself by ripping apart a small binary partner — after deadly spiders. Both redback and black widow females eat the male alive after sex. (In stars, as in spiders, black widows hook up with smaller partners.) Redback and black widows are subcategories of “millisecond pulsars,” neutron stars that spin so fast that they flash Earth every few fractions of a millisecond. But, until now, no one could explain how these nasty stars formed.

Neutron stars are the ultradense remnants of collapsed stars. No wider than a small city, they nevertheless outweigh our sun. Scientists have had to invent all-new physics to explain how matter behaves inside of them. (But unlike black holes, they aren’t quite dense enough to form singularities.) Scientists call them pulsars, because they often appear to telescopes as regularly pulsing light sources. Most spin far faster than normal stars, and their regular rotations can act like clocks ticking away in space.

Related: 7 Surprising Things About the Universe

But a neutron star on its own won’t typically spin fast enough to be a millisecond pulsar, the researchers wrote in the new study. Some external source of energy must kick the pulsar up to its rotational speed. That’s why most millisecond pulsars turn up in binary systems. Astronomers believe that typically, a white dwarf collapses into a neutron star, then at some point down the line starts sucking a stream of matter off its binary twin. The energy from that stream of matter sets the neutron star spinning much faster than it did at birth.

Redbacks and black widows don’t generally fit this model, though. Often the heavier partner in their little binary systems, locked in tight orbits, their intense X-ray beams blast matter off the surfaces of their companion star, knocking that miniature star into space and then sucking it back in with gravity. The masses and energies moving around these systems are very unusual compared with typical millisecond pulsar systems. As a result, the researchers wrote, the normal model for how companion stars accelerate millisecond pulsars doesn’t seem to apply.

In the new paper, published Aug. 14 in The Astrophysical Journal, a team of researchers refined that model. Their paper takes into account the powerful magnetic energy of neutron stars and shows how a neutron star’s magnetism could confine all the matter blasted off the companion star at the neutron star’s north and south poles. That changes the underlying mechanics of the situation, they wrote, and shows that even the smaller partner in redback systems and many black widow systems could accelerate the pulsars to millisecond speeds.

This magnetism theory can’t explain all the black widows we know about, however. But this work should eliminate the need for certain more dramatic theories — like the one published in The Astrophysical Journal in 2015, suggesting that perhaps these sorts of neutron stars are simply born as millisecond pulsars and don’t need any help accelerating.

Originally published on Live Science.

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