... If the Earth is round, the Sun must be north of the equator in northern summer, and south of the equator in northern winter. ... From the Earth's perspective, the Sun is spiraling from north to south and back again along the ecliptic (see figure 7 ). ... The round Earth, at the summer solstice. ... If the rotation axis of the Earth is tilted with respect to its orbit, the equator bobs back and forth, causing the Sun to be north for half the year, and south for the other half (see figure 9 ). ...