On or around June 21 each year, the rays of the sun will be perpendicular to the Tropic of Cancer at 23Â°30' North latitude. This day is known as the summer solstice (more below) in the Northern Hemisphere. The word solstice has its origins in the Latin word solstitium, meaning "the sun stands still". On this day, the earth's "circle of illumination" will be from the Arctic Circle on the far side of the earth (in relation to the sun) to the Antarctic Circle on the near side of the earth. The equator receives twelve hours of daylight, there's 24 hours of daylight at the North Pole and areas north of 66Â°30' N, and there's 24 hours of darkness at the South Pole and areas south of 66Â°30' S.
Summer Solstice Sunrise over Stonehenge
June 20-21 is start of summer in the Northern Hemisphere but simultaneously the start of winter in the Southern Hemisphere. It's also the longest day of sunlight for places in the Northern Hemisphere and the shortest day for cities south of the equator. However, June 20-21 is not the day when the sun rises earliest in the morning nor when it sets latest at night. Interestingly the date of earliest sunrise or sunset varies from location to location.
Summer solstice...... June 20-21 is a very important day for our planet and its relationship with the sun. June 20-21 is one of two solstices, days when the rays of the sun directly strike one of the two tropical latitude lines. June 21 marks the beginning of summer in the northern hemisphere and simultaneously heralds the beginning of winter in the southern hemisphere. In 2009, the solstice occurs and summer begins in the Northern Hemisphere summer begins early on June 21. The earth spins around its axis, an imaginary line going right through the planet between the north and south poles. The axis is tilted somewhat off the plane of the earth's revolution around the sun. The tilt of the axis is 23.5 degrees; and so, thanks to this tilt, we enjoy four seasons. For several months of the year, one half of the earth receives more direct rays of the sun than the other half. (strange but true!). Continue reading........
When the axis tilts towards the sun, as it does between June and September, it is summer in the northern hemisphere but winter in the southern hemisphere. if you want to visit Australia in the winter the time to go is now! Alternatively, when the axis points away from the sun from December to March, the southern hemisphere enjoys the direct rays of the sun during their summer months. June 21 is called the summer solstice in the Northern Hemisphere and simultaneously the winter solstice in the Southern Hemisphere. Around December 21 the solstices are reversed and winter begins in the northern hemisphere. (Confused?)
Earth Lighting Summer Solstice
On June 21, there are 24 hours of daylight north of the Arctic Circle (66.5Â° north of the equator) and 24 hours of darkness south of the Antarctic Circle (66.5Â° south of the equator). The sun's rays are directly overhead along the Tropic of Cancer (the latitude line at 23.5Â° north, passing through Mexico, Saharan Africa, and India) on June 21. Without the tilt of the earth's axis, we would have no seasons. The sun's rays would be directly overhead of the equator all year long. Only a slight change would occur as the earth makes its slightly elliptical orbit around the sun. The earth is furthest from the sun about July 3; this point is known as the aphelion and the earth is 94,555,000 miles phew!)away from the sun. The perihelion takes place about January 4 when the earth is a mere 91,445,000 miles from the sun.
When summer occurs in a hemisphere, it is due to that hemisphere receiving more direct rays of the sun than the opposite hemisphere where it is winter. In winter, the sun's energy hits the earth at oblique angles and is thus less concentrated. During spring and fall, the earth's axis is pointing sideways so both hemispheres have moderate weather and the rays of the sun are directly overhead the equator. Between the Tropic of Cancer and the Tropic of Capricorn (23.5Â° latitude south) there really are no seasons as the sun is never very low in the sky so it stays warm and humid ("tropical") year-round. Only those people in the upper latitudes north and south of the tropics experience seasons.
Of course this is very important on Islay, it allows a good if not short growing season for both creatures and plants. So this week, having been in the wilds for three nights and witnessing sea eagle, Golden eagle and plenty of sea birds as well as foraging and fishing we could easily relate to these long days in the same way as the ancients did. And now from tomorrow the days grow shorter and autumn will be on it's way! Next week I am away finishing the epic London To Paris bike race raising money for leukaemia. So I will miss you....
Other relevant Islay Wildlife and Birding Information Resources:
- Jeremy's News Blog
- Previous Islay nature reports By Jeremy Hastings
- Islay Seasonwatch by Teresa Morris
- Islay Birds blog by Ian Brook
- Islay Birder blog by John Armitage