Why is the Tropic of Cancer important?

The Latitude is a measurement of distance north or south of the equator, the imaginary line that encircles the middle of the planet. The equator's latitude is 0 degrees, and there are 180 imaginary lines (known as parallels) that circle Earth from east to west that run parallel to the equator. And while there aren't proper names for all of them, there are five major ones to know: the equator, the Tropics of Cancer and Capricorn, and the Arctic and Antarctic circles.

The Tropic of Cancer, which is also referred to as the Northern Tropic, is the most northerly circle of latitude on Earth at which the Sun can be directly overhead. This occurs on the June solstice when the Northern Hemisphere is tilted toward the Sun to its maximum extent. It is currently 23°26′12.1″ (or 23.43668°) north of the Equator.

Its Southern Hemisphere counterpart, marking the most southerly position at which the Sun can be directly overhead, is the Tropic of Capricorn. The positions of these two circles of latitude (relative to the Equator) are dictated by the tilt of Earth's axis of rotation relative to the plane of its orbit, and since the tilt changes, the location of these two circles also changes.

When this line of latitude was named in the last centuries BC, the Sun was in the constellation Cancer (Latin for crab) at the June solstice, the time each year that the Sun reaches its zenith at this latitude. Due to the precession of the equinoxes, this is no longer the case; today the Sun is in Taurus at the June solstice. The word "tropic" itself comes from the Greek "trope", meaning turn (change of direction, or circumstances), inclination, referring to the fact that the Sun appears to "turn back" at the solstices.

There are approximately 13 hours, 35 minutes of daylight during the summer solstice. During the winter solstice, there are 10 hours, 41 minutes of daylight.

The Tropic of Cancer's position is not fixed but constantly changes because of a slight wobble in the Earth's longitudinal alignment relative to its orbit around the Sun. Earth's axial tilt varies over a 41,000-year period from 22.1 to 24.5 degrees and currently resides at about 23.4 degrees. This wobble means that the Tropic of Cancer is currently drifting southward at a rate of almost half an arcsecond (0.468″) of latitude, or 15 meters, per year (it was at exactly 23°27′N in 1917 and will be at 23°26'N in 2045).

At this place where the Mexican Highway crosses the Tropic of Cancer, the latitude is marked with precision and where the annual drift between the years 2005 and 2010 can be appreciated.

Sources -

1. Wikipedia
2. HowStuffWorks
3. PHP Science Labs

Eugene Parker - the astrophysicist’s discoveries about the Sun led NASA to name a new solar mission after him

Born on 10 June 1927 in Houghton, Michigan, Eugene Parker is an astrophysicist who predicted the existence of the solar wind. Parker received a BS in physics from Michigan State University in 1948 and a PhD from Caltech in 1951. He then taught at the University of Utah, until accepting a position at the University of Chicago in 1955.

Parker began his investigation into the possibility of a solar wind because of the observations of comets and their tails, which always pointed away from the Sun. His mathematical calculations led to his proposing that a stream of high-energy charged particles originates in the Sun’s upper atmosphere and flows at supersonic speeds throughout the solar system and beyond.

The paper he submitted to the Astrophysical Journal was initially rejected. Parker appealed to the journal’s editor, Subrahmanyan Chandrasekhar, who overruled the reviewers and published the paper in 1958. Parker’s theory was validated four years later by measurements taken by the Mariner 2 probe on its way to Venus.

Sixty years later, NASA launched the Parker Solar Probe, the first NASA spacecraft to be named for a living person. Over his career, Parker studied other phenomena, such as cosmic rays and the magnetic fields of galaxies.

Such terms as the Parker instability, the Parker equation, and the Parker limit attest to his contributions to the field. In recognition of his work, he received numerous awards and honours, including the 1978 George Ellery Hale Prize, the 1989 National Medal of Science, the 1992 Royal Astronomical Society Gold Medal, the 1997 Bruce Medal, the 2003 Kyoto Prize, and the 2003 James Clerk Maxwell Prize.

Source - Physics Today