2018 June 21st at 10.07 GMT
The longest day of the year!
The Earth’s axis is tilted by 23.4 degrees and so the plane of the Earth’s equator is tilted with respect to the plane of the Earth’s orbit around the Sun; referred to as the ecliptic.
The solstice is an astronomical event, caused by Earth’s tilt on its axis and its motion in orbit around the sun. Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly. At the June solstice, Earth is positioned in its orbit so that our world’s North Pole is leaning most toward the sun. As seen from Earth, the sun is directly overhead at noon 23.5 degrees north of the equator, at an imaginary line encircling the globe.
All locations north of the equator have days longer than 12 hours at the June solstice. Meanwhile, all locations south of the equator have days shorter than 12 hours.
This means that during the Northern Hemisphere summer, the Sun illuminates the Northern Hemisphere more than the Southern Hemisphere. During this time we receive more hours of sunlight and the longest day is known as the summer solstice. During the winter, the Sun illuminates the Southern Hemisphere more than the Northern Hemisphere because the North Pole is titled away from the Sun. During this time we receive fewer hours of sunlight and the shortest day (and longest night) is known as the winter solstice.
But at two points in the year the Sun will illuminate the Northern and Southern Hemispheres equally – these are known as the equinoxes: the autumnal equinox in October and vernal equinox in March. It’s the moment in which the plane of Earth’s equator passes through the centre of the Sun’s disk or the moment that the Sun passes the celestial equator from the Northern to the Southern Hemisphere or vice a versa. On these dates, there are approximately equal hours of daylight and darkness.
Where does the word ‘equinox’ come from?
The word equinox comes from the Latin aequinoctium meaning ‘equal night’.
Where does the word ‘solstice’ come from?
The world ‘solstice’ comes from the Latin solstitium meaning ‘Sun stands still’ because the apparent movement of the Sun’s path north or south stops before changing direction.
Why don’t the solstices and equinoxes occur on the same days annually?
The Earth takes approximately 365¼ days to go around the Sun. This is why we have a leap year every four years to add another day to our calendar; and so that there is not a gradual drift of date through the seasons.
For the same reason the precise time of the equinoxes are not the same each year, and generally will occur about six hours later each year, with a jump of a day (backwards) on leap years.
Enjoy the day!