Our spherical planet has two hemispheres divided at the equator. Those living in the north hemisphere experience the opposite season as those living in the south hemisphere at any given time of year. For example: New Zealand summer/Britain winter, USA spring/Australia autumn, etc. Is it possible for a planet to have the same seasons in opposite hemispheres? If so, how?
Yes it is possible, if the planet has little axial tilt but an eccentric orbit. Then summer will be when it is closest to its sun, and winter will be when it is furthest away, which will be the same all over the planet — at the equator as well as in higher northern and southern latitudes.
Of course. Just make sure the axial tilt is zero. Basically, 99% of our weather changes are due to the angle of solar incidence, and only a small amount is due to the distance Earth is from the Sun (elliptical orbit). Earth's axial tilt is roughly 23.44 degrees. For a given latitude, a little trigonometry will show how the angle of solar incidence (draw a line from the sun and see what angle it makes with the ground) changes as that axial tilt faces towards or away from the sun -- and note that "towards" in the Northern Hemisphere means "away" in the Southern Hemisphere.
Thanks to Matthew for reminding me that there's a significant change in the number of hours of daylight as the orientation of the axial tilt changes. That's at least as important as the angle of incidence of the sun's rays.
Expanding on the other answers here, let's start with an overview of why there are seasons. I really like this description:
We have seasons because the earth is tilted (wonky) as it makes its yearly journey around the sun. The Earth's axis is tilted at an angle of 23.5 degrees. This means that the Earth is always "pointing" to one side as it goes around the Sun. So, sometimes the Sun is in the direction that the Earth is pointing, but not at other times. The varying amounts of sunlight around the Earth during the year, creates the seasons.
Here's a visualization of the orbital tilt and seasons from Wikipedia.
The Earth stays roughly the same distance from the Sun because it has a (roughly) circular orbit. But there are a lot of different paths that a planet can take to orbit a star. When a planet moves farther from the star, the planet gets colder (winter). When the planet gets closer to the star, the planet gets warmer (summer). The relevant Wikipedia article is complicated but this GIF shows several different orbits.
Imagine you're on the planet below. If the planet isn't tilted then the two hemispheres would experience the seasons at the same time. Check out this question for more detail.
Other answers are right at pointing at eccentricity as the most likely cause for global seasons. However, there is another possible cause: multiple stars.
If the planet were orbiting a double star, the distance to each star might change as the stars orbit each other. If they have different mass and bright, the total amount of heat the planet gets may change producing seasons.
A planet orbiting around one of the stars of the binary system could also get a changing amount of heat from the other star.
Interestingly, binary stars could lead to a wide range of changes in radiation and even color of light that could make interesting plot devices.
Zero tilt, elliptical orbit are the requirements but there something else that is important...winds.
Winds blow due to pressure differentials due to mainly two reasons on earth, thermal insolation differences that drive sea-land vice verse winds and coriollis forces that drive planetery winds.
In the above situation even with uneven land-mass distribution you may find most winds blowing towards the poles...causing thinning of atmosphere near equater...people can correct me in the comments on this prediction I’ll be happy to edit.
There are five ways that this could happen:
- a very eccentric orbit with little/no axial tilt
- a pulsating star with little/no eccentricity or tilt
- the majority of the planet's energy comes from a star orbiting a black hole sharing a similar orbit with the same semi-major axis but has different eccentricity to the planet
- the planet is in a system with binary stars and little tilt/eccentricity
- the "planet" is a moon of a gas giant that gets heat from tidal locking
As an example, Venus with its extremely thick atmosphere over a rocky world yields extremely similar temperatures and seasons over the whole planet.
Multiple circulation cells, very high density atmosphere, and thick clouds/dust to absorb insolation all conspire to make the surface conditions relentlessly the same.
Venus also has very little axial tilt, so it wouldn't have much for seasons even if it had only 1 standard atmospheric pressure. But you might adapt your story with Venus in mind.