Scope on the Skies
By BOB RIDDLE
Astronomy throughout the year
On the morning of November 11, as the Sun rises, the innermost planet Mercury will transit the Sun in an event that will be visible across the continental United States. The entire transit will be visible from the East Coast, and the transit will be in progress farther west as the Sun rises. First contact will be at 12:35 UT (7:35 a.m. EST) as the leading edge of Mercury contacts the edge of the Sun (see Figure 1). What does a transit of the Sun look like? Only the two inner planets, Mercury and Venus, may transit the Sun. The planet will appear as a small dot moving from left to right across the Sun’s disk (see Safety note). A transit consists of four stages: (I)the leading edge of Mercury’s disk contacts the edge of the Sun’s disk; (II)the trailing edge of Mercury disk is within the Sun’s disk; (III)the leading edge of Mercury’s disk contacts the opposite side of the Sun’s disk; and (IV)the trailing edge of Mercury’s disk leaves the Sun’s disk.
As we view from Earth, Mercury will pass in between the Earth and the Sun much like our Moon during a solar eclipse. In both situations, the transit, or eclipse, occurs when there is a node crossing at inferior conjunction for either Mercury or Venus or at a new Moon phase. A node crossing refers to the point where two orbits intersect. The Earth’s orbit around the Sun (typically referred to as the ecliptic) is used as the reference. Any other object orbiting the Sun with its respective orbital path inclined or tilted away from the ecliptic will have a node crossing at the point where that orbit intersects the Earth’s orbit. Mercury, for example, has an inclination of around 7°. There are two node crossings: an ascending node and a descending node. If an inner planet’s solar transit takes place during a node crossing, the transit will last for a long time.
Transits of the inner planets are somewhat rare occurrences, with approximately 13 Mercury transits each century occurring only in May or November. The most recent Mercury transit was during May 2016. After this year, the next Mercury transit will not be until May 2032. On the other hand, Venus transits are even more rare, occurring as pairs of transits in June or December. Each pair of Venus transits happen every 115 or 121 years. The most recent pair of Venus transits were 2004 and 2012, and the next pair will not be until next century during December of 2117 and 2125. Solar transits of the two inner planets occur in the same two months following a pattern or cycle that is determined by the orbital periods of the Earth and the inner planets and the respective individual orbital periods of each planet.
Fifteen months after the launch of the Parker Solar Probe (August 2018), the solar orbiter will make its second flyby of the inner planet Venus as it moves toward completion of the third orbit around the Sun. The probe is designed to orbit the Sun close enough to allow scientists to gather data about the Sun’s corona, solar wind, and the processes involved. The Parker Solar Probe orbits the Sun in approximately 88 days, and at speeds of over 640,000 km/h (400,000 mph). The mission has a total of 24 planned orbits over a six-year period. At its closest perihelion to the Sun, the probe will be less than 6,437,376 km (4,000,000 mi.) away (see Resources).
On December 22 at 11:19 p.m. EST, the Sun, in its apparent eastward motion, crosses the ecliptic at the celestial coordinates of 18 hours right ascension and 23.5° south declination. This position marks the southernmost declination for the Sun within the constellation of Sagittarius the Archer. At the time of the node crossing, the Sun will be moving south. This time also marks the end of Northern Hemisphere fall and the beginning of winter. In the Southern Hemisphere, it will be the opposite, with winter ending and summer beginning. The Sun’s apparent motion is caused by the Earth’s revolution around the Sun; each day the Sun moves eastward along the ecliptic approximately 1°, or 2.6 million km (1.603 million miles). See Resources for Star Charts.
On December 26, the new Moon will pass directly between the Earth and the Sun, setting up the conditions necessary for a solar eclipse visible from Saudi Arabia eastward across southern India and Indonesia. An eclipse of the Sun, and the Moon, involves a timing coincidence where the Moon, at either new or full phase, crosses the Earth’s orbit and passes either through the Earth’s shadow or between the Earth and the Sun. This event happens due to the tilted orbital path of the Moon in relation to Earth’s orbit. The Moon’s orbit intersects the ecliptic. As the Moon orbits around the Sun with the Earth, it will cross the plane of the ecliptic at least twice each month with an ascending node and a descending node.
The solar eclipse on December 26 will be a total solar eclipse; however, at mid-eclipse the Sun will not be completely blocked by the new Moon. While at mid-eclipse, when the Moon is centered on the Sun, there will be a ring of the Sun around the silhouette of the Moon. This “ring of fire” is known as the annulus, and therefore, this eclipse will be an annular solar eclipse. As with all solar eclipses, the closer one is to the center line of the Moon’s shadow path across the Earth’s surface, the greater percentage of the Sun will be covered. Viewing of this solar eclipse will range from China to parts of Australia, with the centerline and maximum Sun coverage in Bombay, India. Although this eclipse will not be visible from the continental United States, it will offer an opportunity for students to learn about the countries the Moon’s shadow path will cross (see Resources).
Excel Spreadsheet Transit Calculator—
Free Star Charts (PDF)—
Interactive Transit Map—
Mercury Transit Calculator—
Parker Solar Probe (Johns Hopkins University)—
Parker Solar Probe (NASA)—
Some Local Mercury Transit Times—
Where is the Parker Solar Probe?—
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