Earth approaches closest point to the sun
As the new year dawns, the Summer Triangle plunges toward the western horizon. Cygnus (SIG-nuss), the Swan, wings south along the Milky Way with the geese seeking warmer climes for the winter. For us, the days are slowly starting to lengthen. The latest sunrise was yesterday (7:33 a.m. over a flat horizon in Tupper Lake). This morning it was a second earlier, on Jan. 15, sunrise will occur at 7:30 a.m.. After this, it will change more rapidly, to 7:15 a.m. on Feb. 1 and 6:57 a.m. on Feb. 15. The evenings have been getting longer since Dec. 9 when the sun set at 4:18:51 p.m. in Tupper Lake to this evening’s 4:31:51 p.m. On Feb. 1, the sunset will occur at 5:08 p.m. Of course winter’s grip of cold is still strengthening, but the slow return of light reminds us that summer will come again!
However long they can seem, our northern hemisphere winters are shorter than our summers. The Vernal Equinox will arrive at 6:29 a.m. on March 20, 2017, 178.8 days since the Autumnal Equinox last September. But it will take 186.4 days to get back to the Autumnal Equinox at 8:02 p.m. on Sept. 22, 2017. This asymmetry in the year is due to the fact that the Earth’s orbit is an ellipse, not a circle.
Tomorrow, Earth will be at its closest point to the Sun, perihelion, at 9:17 a.m. The distance will be 91,404,319 miles. At 4:11 p.m. on July 3, Earth will be at aphelion, its farthest from the Sun at 94,505,901 miles. That’s only a 3.3% difference in distance, but it also results in a change of Earth’s orbital speed! Johannes Kepler published his discovery of this in 1609. At perihelion, tomorrow, Earth will be moving at 67,753 mph along its orbit. Then Earth will begin to slow as it moves farther from the sun. At aphelion in July, it will have slowed to 65,529 mph, allowing us northerners an extra seven-and-a-half days of summer. So step out into the cold to stargaze knowing that July is coming at almost 68 thousand miles per hour!
If you have a low southwestern horizon and observe early after sunset, locate the “lonely star” Fomalhaut (FOAM-a-lot … as in “if you pour beer too fast it will foam a lot!) just below and left of Venus as shown in Figure 1. The brightest star in Piscis Austrinus (pis-SIS aus-STREE-nus), the southern fish, its name reflects its position in the fish’s mouth. It is in the midst of the “watery constellations” south of Pegasus that include Pisces (PIE-seas, the fish), Cetus (SEAT-uss, the sea monster) with the bright star Deneb Kaitos (DEN-ebb KYE-tose, rhymes with dose) and Aquarius (the water bearer).
As well as being one of the southernmost bright stars we can see from the Adirondacks, Fomalhaut is also the first star to have an orbiting planet photographed. Figure 2 shows the Hubble Space Telescope image of Fomalhaut with the bright star blocked to allow its surroundings to be seen, although light reflecting off surrounding dust and ice creates a diagonal swath of light across the image. The planet, Fomalhaut b, is on a 2000-year-long elliptical orbit with its closest approach to the star five times Saturn’s distance from the Sun. There is also a huge ring of debris surrounding Fomalhaut that the planet will cross in about 15 years. If it is in the same plane as the ring, icy and rocky debris should collide with it giving astronomers a sense of its solidity and atmosphere. In the early history of our solar system, it is theorized that some large planets that were formed close to the sun were ejected into larger orbits by the tidal actions of other large planets. This may be what put Fomalhaut b into such a large orbit.
Speaking of planets around other stars, NASA’s Kepler Mission, launched in March, 2009, started searching for extrasolar planets on May 12, 2009. The grey area on Figure 1, just above the right wing of the swan shows the area of the sky it searched. It was used to find and confirm 2,331 exoplanets by the end of its mission in 2012. Its mission was then extended to 2016 under the name K2 that has so far found and confirmed 173 additional exoplanets with 458 candidates added to the 4,696 Kepler candidates still requiring confirmation.
Fomalhaut also marks an important line on our celestial coordinate system. Astronomers have a grid on the sky with coordinates of Right Ascension (celestial longitude) and Declination (celestial latitude). The declination is measured from the Celestial Equator (the projection of Earth’s Equator onto the sky) in positive degrees (north) and (negative) south. Astronomers use signs for north and south instead of N and S because we do many calculations with coordinates. Right Ascension is measured from the “first point of Aries,” or the Vernal Equinox. That point is no longer in Aries because of the precession of the equinoxes, but it retains the ancient name as do the Tropics of Cancer and Capricorn (that should now be the Tropics of Taurus and Sagittarius … but it’s too much to change all the globes and maps!!). Physically, this is the point where the Ecliptic, the annual path of the Sun (and moon and planets) crosses the Celestial Equator. When the sun is at this point, it’s crossing from south of the Celestial Equator to north of it. For us, that’s the first day of spring! At its average rate of 1 degree per day along the Ecliptic, the sun will take about 79 more days to get there. You can watch the progress as the stars slip about 1° to the west at sunset each night until the Vernal Equinox will set with the sun.
Right Ascension is measured in hours to the east from the Vernal Equinox because the sky goes by at a rate of 15°/hour (360 degrees/24 hours). The celestial meridian one hour west marking 23h falls barely to the east of Fomalhaut, giving it a Right Ascension of 22h 57.658m (hours and minutes with 60 minutes per hour). This meridian is also quite close to the right side (west) of the Great Square between Markab (MAR-cobb) and Sheat (SHE-ought). The prime meridian at zero hours is most closely marked by Omega Piscium (PISS-ee-um), just left (east) of the Ringlet of Pisces just below the Great Square … but for naked-eye observing, the left side of the Great Square from Algenib (al-JEN-ebb) to Alpheratz (AL-fur-atz) is close enough to give a sense of our celestial grid. The top and bottom of the Great Square are at roughly 30 degree and 15 degree, respectively.
The astronomers of the Adirondack Public observatory will be happy to focus their telescopes on many stars and wonders from our Roll Off Roof Observatory (RORO) above Little Wolf Pond in Tupper Lake. Currently, we’re open to the public on the first and third Fridays of each month, weather permitting of course. For updates and notices, check out our website at adirondackpublicobservatory.org and our Facebook page. On our public observing days you can also call the RORO (Roll Off Roof Observatory) at 518-359-6317 to talk with one of our astronomers. Observing starts about one half hour past sunset.