THE SUMMER SOLSTICE – WHAT IS IT?

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THE SUMMER SOLSTICE – WHAT IS IT?

What is the summer solstice all about?   Is it necessary for us to be aware of what is behind it? Should we be interested in the solstice?

The summer solstice occurs when the tilt of a planet’s semi-axis, in either the northern or the southern hemisphere, is most inclined toward the star (sun) that it orbits. Earth’s maximum axil tilt toward the sun is 23° 26′. This happens twice each year, at which times the sun reaches its highest position in the sky as seen from the north or the South Pole.

The summer solstice occurs during a hemisphere’s summer. This is the northern solstice in the northern hemisphere and the southern solstice in the southern hemisphere. Depending on the shift of the calendar, the summer solstice occurs sometime between June 20 and June 22 in the northern hemisphere and between December 20 and December 23 each year in the southern hemisphere.

When on a geographic pole, the sun reaches its greatest height, the moment of solstice; it can be noon only along that longitude which at that moment lies in the direction of the sun from the pole. For other longitudes, it is not noon. Noon has either passed or has yet to come. Hence the notion of a solstice day is useful. The term is colloquially used like midsummer to refer to the day on which solstice occurs. The summer solstice day has the longest period of daylight – except in the Polar Regions, where daylight is continuous, from a few days to six months around the summer solstice.

Worldwide, interpretation of the event has varied among cultures, but most recognize the event in some way with holidays, festivals and rituals around that time with themes of religion or fertility. The solstice is also known as the birthday of the sun, like the queen the sun celebrates twice a year.

Solstice is derived from the Latin words sol (sun) and sistere (to stand still), because at the solstices, the Sun stands still in declination; that is, the seasonal movement of the Sun’s path (as seen from earth) comes to a stop before reversing direction.

A solstice is an astronomical event that occurs twice each year as the Sun reaches its highest or lowest excursion relative to the celestial equator on the celestial sphere. The solstices, together with the equinoxes, are connected with the seasons. In many cultures the solstices mark either the beginning or the midpoint of winter and summer.

At latitudes in the temperate zone, the summer solstice marks the day when the sun appears highest in the sky. However, in the tropics, the sun appears directly overhead (called the subsolar point) some days (or even months) before the solstice and again after the solstice, which means the subsolar point occurs twice each year.

The term solstice can also be used in a broader sense, as the date (day) when this occurs. The day of the solstice is either the longest day of the year (in summer) or the shortest day of the year (in winter) for any place outside of the tropics.

For an observer on the North Pole, the sun reaches the highest position in the sky once a year in June. The day this occurs is called the June solstice day. Similarly, for an observer on the South Pole, the sun reaches the highest position on December solstice day. When it is the summer solstice at one Pole, it is the winter solstice on the other. The sun’s westerly motion never ceases as the Earth is continually in rotation. However, the sun’s motion in declination comes to a stop at the moment of solstice. In that sense, solstice means “sun-standing”. This modern scientific word descends from a Latin scientific word in use in the late Roman republic of the 1st century BC: solstitium. Pliny uses it a number of times in his Natural History with a similar meaning that it has today. It contains two Latin-language morphemes, sol, “sun”, and -stitium, “stoppage”. The Romans used “standing” to refer to a component of the relative velocity of the Sun as it is observed in the sky. Relative velocity is the motion of an object from the point of view of an observer in a frame of reference. From a fixed position on the ground, the sun appears to orbit around the Earth.

To an observer in an inertial frame of reference, the planet Earth is seen to rotate about an axis and revolve around the Sun in an elliptical path with the Sun at one focus. The Earth’s axis is tilted with respect to the plane of the Earth’s orbit and this axis maintains a position that changes little with respect to the background of stars. An observer on Earth therefore sees a solar path that is the result of both rotation and revolution.

The component of the Sun’s motion seen by an earthbound observer caused by the revolution of the tilted axis – which, keeping the same angle in space, is oriented toward or away from the Sun – is an observed daily increment (and lateral offset) of the elevation of the Sun at noon for approximately six months and observed daily decrement for the remaining six months. At maximum or minimum elevation, the relative yearly motion of the Sun perpendicular to the horizon stops and reverses direction.

Outside of the tropics, the maximum elevation occurs at the summer solstice and the minimum at the winter solstice. The path of the Sun, or ecliptic, sweeps north and south between the northern and southern hemispheres. The days are longer around the summer solstice and shorter around the winter solstice. When the Sun’s path crosses the equator, the length of the nights at latitudes +L° and -L° are of equal length. This is known as an equinox. There are two solstices and two equinoxes in a tropical year.

The seasons occur because the Earth’s axis of rotation is not perpendicular to its orbital plane (the “plane of the ecliptic”) but currently makes an angle of about 23.44° (called the “obliquity of the ecliptic “), and because the axis keeps its orientation with respect to an inertial frame of reference. As a consequence, for half the year the Northern Hemisphere is inclined toward the Sun while for the other half year the Southern Hemisphere has this distinction. The two moments when the inclination of Earth’s rotational axis has maximum effect are the solstices.

At the June solstice the subsolar point is further north than any other time: at latitude 23.44° north, known as the Tropic of Cancer. Similarly at the December solstice the subsolar point is further south than any other time: at latitude 23.44° south, known as the Tropic of Capricorn. The subsolar point will cross every latitude between these two extremes exactly twice per year.

Also during the June solstice, places on the Arctic Circle (latitude 66.56° north) will see the Sun just on the horizon during midnight, and all places north of it will see the Sun above horizon for 24 hours. That is the midnight sun or midsummer -night sun or polar day. On the other hand, places on the Antarctic (latitude 66.56° south) will see the Sun just on the horizon during midday, and all places south of it will not see the Sun above horizon at any time of the day. That is the polar night. During the December Solstice, the effects on both hemispheres are just the opposite. This also allows the polar sea ice to increase its annual growth and temporary extent at a greater level due to lack of direct sunlight.

The concept of the solstices was embedded in ancient Greek celestial navigation. As soon as they discovered that the Earth is spherical they devised the concept of the celestial sphere, an imaginary spherical surface rotating with the heavenly bodies fixed in it (the modern one does not rotate, but the stars in it do). As long as no assumptions are made concerning the distances of those bodies from Earth or from each other, the sphere can be accepted as real and is in fact still in use.

The stars move across the inner surface of the celestial sphere along the circumferences of circles in parallel planes perpendicular to the Earth’s axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole. The Sun and the planets do not move in these parallel paths but along another circle, the ecliptic, whose plane is at an angle, the obliquity of the ecliptic, to the axis, bringing the Sun and planets across the paths of and in among the stars.

The term heliacal circle is used for the ecliptic, which is in the center of the zodiacal circle, conceived as a band including the noted constellations named on mythical themes. Other authors use Zodiac to mean ecliptic, which first appears in a gloss of unknown author in a passage of Cleomedes where he is explaining that the Moon is in the zodiacal circle as well and periodically crosses the path of the Sun. As some of these crossings represent eclipses of the Moon, the path of the Sun is given a synonym, the ekleiptikos (kuklos) from ekleipsis, “eclipse”.

Kathy Kiefer

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