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If the Earth's axial tilt were 0˚, what would "seasons" on Earth be like? What would the day/night cycle be like?
If the Earth's axial tilt were 0˚, every day would be like the equinox - with equal day and night. The only source of seasons would the change in distance between the Earth and Sun throughout the year (147 to 153 million km). This means summer would occur in January everywhere on Earth. You would still have colder climates as you increase in latitude, can you explain why?