Starwatch: March equinox is upon us as sun crosses celestial equator

Length of day and night roughly equal as longer summer days draw near

The sun crosses the celestial equator this week, meaning that the March equinox is upon us.

The celestial equator is the projection of the Earth's equator up into the sky. Because Earth rotates on a tilted axis, which always points in the same direction, our orientation to the sun changes throughout the year. When we are in the hemisphere tilted towards the sun, our parent star appears higher in the sky than the celestial equator and we experience summer with its longer days. Likewise, when the axis is pointed away, the sun never rises as high as the celestial equator and we experience winter with shorter days.