The Vernal or Spring Equinox occurs today, March 20 at 2:58 pm PDT: Welcome to Spring!
Equinoxes occur at a time when the Earth is not titled either towards or away from the sun – yesterday (in Winter) the Northern Hemisphere was titled away from the sun and tomorrow it will be titled towards the sun (Spring). This corresponds to when the sun passes directly over Earth’s equator and crosses the celestial equator in the sky. You can use Stellarium to see that the sun is located in constellation Pisces and how it moves from south of the equator to north of the equator.
Crossing the celestial equator is important for determining right ascension – the celestial equivalent of longitude. The zero point for right ascension is defined to be the point where the sun crosses the celestial equator at the vernal equinox. This is also known as the First Point of Aires which is a mystery because the we just saw that the sun is in Pisces.
Hipparchus defined the First Point of Aries in 130 BCE when the sun was located in the constellation Aries during the spring equinox. Due to the Earth’s precession that point has been moving westward at a rate of approximately 1 degree every 72 years and is now located in Pisces – mystery solved.
Bonus: There is also a full “super” moon tonight – the last supermoon of 2019 – and the forecast is for clear skies so get out and have a look.