The 23.5 degree tilt is responsible for the seasons. If the earth had no tilt there would not be seasons. If the earth was tilted by 90 degrees the seasonal changes would be at the most extreme. The Earth's pole would point directly at the sun at a point on the track around the sun. As the Earth revolves around the Sun the pole would alternate twice each year between pointing directly at the sun and being perpendicular to the sun. When the pole points directly at the sun there would not only be 24 hours of sunlight but the sun angle would be very high. This would produce very hot temperatures at one pole while at the same time the other pole would be in complete darkness with extreme cold. One hemisphere will be in mostly darkness while the other would be in mostly sunlight for many weeks. Three months later the pole will be perpendicular to the sun. At this point both poles would have a low sun angle and the current equatorial regions would have 12 hours of sun and 12 hours of night each day. The sun would rise in the northern Earth sky and set in the south. Three months later the poles will switch from 6 months earlier. The opposite pole will now have a high sun angle and 24 hours of light. 3 months later the pole will once again be perpendicular to the sun. Because the Earth is on the other side of the sun the sun will now rise in the southern Earth sky and set in the north. Three months later the year will be completed and the cycle would start again. During the course of the year all locations on earth would alternate between extreme heat and extreme cold. Each pole would alternate between 6 months of light and dark. The sun would always be out at the equator but the noon sun angle would change dramatically during the course of a year.