Old Man Winter is on its last legs as the Vernal Equinox arrives Saturday, March 20th at 5:37 a.m. EDT, with the start of spring.  The first signs of spring warmth have been showing up across much of the country already this month as people, plants, and animals are seeing a gradual ending to winter's chill. For millennia, people have been using the astronomical calendar to track all four seasons. The natural rotation of the Earth around the Sun forms the basis for this calendar.  The vernal or spring equinox is one of two days of the year when the sun is directly overhead if you are at the equator.  The amount of sunlight on the equinox is the same for the northern and southern hemisphere. While the word equinox means “equal night” in Latin, by time of the equinox in the northern hemisphere the day is already a bit longer than the night.  This unequal length of day and night on a day that should have them equal is due to atmospheric refraction and the way we define sunrise and sunset. Refraction of the sun’s light through the earth’s atmosphere causes the sun to appear slightly higher in the sky than it actually is. Refraction is the optical phenomenon that bends the sun’s light as it passes through Earth’s atmosphere.  That means that the first light of a sunrise gets to our eyes while the sun is still below the horizon. The way we define sunrise and sunset is also responsible for the disproportionate day parts.  We define sunrise as the moment the sun’s upper edge appears on the horizon and sunset at the moment the the sun’s upper edge disappears below the horizon.  This way of defining a day and the optical phenomenon of refraction are responsible for a non-equal, equinox. Day and night hours are equal two times a year and we refer to that time as the “equilux”.  The equilux happens a few days before the spring equinox, and a few days after the autumn equinox. Springtime does bring warmer temperatures to the northern hemisphere, but it's often a gradual occurrence.  Much of the warmth from longer days and higher sun angles during early spring is used to warm the earth and water in the northern hemisphere so it takes a while for those warmer temperatures to take hold. This weekend, temperatures will resemble spring readings across much of the eastern sections of the nation. Enjoy and check back with WeatherNation for the latest forecasts.