At present Earth is tilted 23.5 degrees on its axis. This is the primary reason for the seasons. When the Earth is tilted toward the sun the Northern Hemisphere is in the warm season and when the Earth is tilted away from the sun the Northern Hemisphere is in the cool season. Since the Earth revolves at a predictable and relatively steady rate around the sun the cycle repeats itself every year. In the warm season the sun is higher in the sky and it is warmer and in the cool season the sun is lower in the sky and it is cooler.

What if though the Earth was not tilted? In this case the plane of the Earth's poles would always be perpendicular to the sun. The sun would always be just on the horizon 24 hours a day on every day at the poles. Every day would be like what it currently is on the equinox since every location on Earth would have about a 12 hour sunlight days and the noon sun angle would be about the same every day. There would no longer be season as we know them. The temperature and precipitation pattern would not vary much. It would still be warm at the equator and cold at the poles. The most profound impact on temperatures would be at the poles. Instead and dark and frigid temperatures in the winter and warmer and constant sun in the summer, the polar areas would have much more uniform temperatures year round and the sun would always be low on the horizon. Across the Earth it would be like it is in the middle of fall or spring but it would last all year every year. Areas today that have wet, dry, warm and cold seasons would have a fairly constant weather all year whether it be wet, dry, warm and/or cold.

Would there be any differences at all? There would still be some slight changes during the year even though there would not be seasons as we know them now. The Earth-Sun distance does vary during the year. Currently the sun is closest to Earth in the Northern Hemisphere winter and further away in summer. With no tilt, this change in Earth-Sun distance during the year would produce a slight impact on the weather pattern. It must be emphasized the impact would be small since the Earth-Sun distance is not significantly different during the year (91.4 million miles in January compared to 94.5 million miles in July). Earth having tilt has a far greater impact on the weather pattern but without any tilt the Earth-Sun distance would have the dominant impact on season.