Do you know the difference between an equinox and solstice?
Astronomical seasons are when we mark the change from winter to spring, spring to summer, etc. They are based on solstices and equinoxes which equate to the tilt of the earth. There is a winter and summer Solstice, then a fall (autumnal) and spring (vernal) equinox. There are also "meteorological seasons," but we will get into that in a future blog.
The winter solstice is when the earth is closest to the sun; however, the tilt of the Earth is away from the sun in the northern hemisphere. This is opposite in the summer when the Earth is farther from the sun, but the tilt of Earth's axis is towards the sun. This tilt causes varying levels of direct sunlight based on the time of year and is the reason why we have seasons. For example, when the northern hemisphere is tilted away from the sun in the winter, the area receives less direct sunlight and results in colder temperatures. We perceive the sun lower in the sky and see less daylight time. In the summer, this is opposite with a higher sun angle, more direct sunlight, and a longer day. The distance to the sun does not really affect temperatures on Earth because there isn't much difference between our closest point and farthest point in our orbit (only about 5 million kilometers, not much in astronomical terms).
On the day of the summer solstice (usually falls between June 20 - 22), the sun is directly over the Tropic of Cancer, 23.5 degrees north latitude (also the tilt of the earth). During the winter solstice (usually falls between December 21 - 22), the sun is directly over the Tropic of Capricorn, 23.5 degrees south latitude (Over South America). This may not seem like a large difference in the grand scheme of things, but our sun angle changes 47 degrees from winter to summer. If you look up in the sky during the summer, imagine about 47 degrees towards the horizon, this is where the sun will be during the winter solstice.
Now we'll change directions slightly and get into equinoxes. As we head into spring and fall, which are the transition seasons, the sun is directly over the equator (center of the earth) and the amount of sunlight is distributed evenly across the earth. So, 2 days a year, the equator has an even 12 hours of sunlight and 12 hours of darkness. Typically, the autumnal equinox is September 22 or 23, while the vernal equinox is March 20 - 21.
One thing to clear up, the earth is not physically tilting back and forth, it is in a stationary position on an axis and in orbit around the sun. So, it is always tilted 23.5 degrees, but only in one direction as it moves around the orbit. The image below gives a great representation of this.
This image was found at https://www.weather.gov/cle/seasons