Tag Archives: Sun

Transit of Mercury: A Unique Astronomical Event November 11, 2019

Venus and Mercury transits occur when these worlds, which orbit between the Earth and the Sun, can be seen to pass directly in front of the solar disk and transit across the face of our star. Why don’t we experience a transit of Venus or Mercury every time they pass between the Earth and the Sun (called inferior conjunction)? It all has to do with the orbits of these planets and our ever-changing viewing angle. Most of the time Venus and Mercury pass above or below the solar disk as seen from the Earth. This concept is simply stated here, but it took the greatest astronomical minds of the past to solve this great mystery. The process took much observation, dedication and deduction to determine the solar system design and the celestial mechanics that govern its motion.

Venus transits are rare astronomical events. They always occur in pairs, eight years apart. We last experienced transits of Venus in 2004 and 2012. Their immediate predecessors occurred back in 1884 and 1882. And the next pair won’t be until 2117 and 2125!

However, another planet can transit the Sun—Mercury. Though not as rare as Venus transits, transits of Mercury occur 12-13 times per century. The last one occurred on May 9, 2016 and it was observed in its entirety locally. The Mercury transit prior to that occurred on November 8, 2006, but in Southern New England we were clouded out. Unfortunately, the next one visible here after the upcoming November 11 event won’t be until May 7, 2049.   

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Solar Variability: Our Inconstant Sun

I went to bed last night wondering where the Sun had gone. Then it dawned on me. A little astronomy humor to begin a serious discussion on our life-giving star—the Sun.

Our Sun coalesced out of a vast cloud of gas and dust some 100 times the size of our solar system roughly five billion years ago. Gravity contracted this mass until the core of this proto-star reached about 24.5 million degrees Fahrenheit. At that time nuclear fusion began, converting about 600 million tons of hydrogen to helium per second. This process has been ongoing since then.

Most folks take for granted that the Sun rises and sets every day due to the Earth’s almost 24-hour rotation. The only change most people recognize is the seasonal cycles caused by the fixed 23.5 degree tilt of our planet’s axis in its orbital path about the Sun. Reference this video to refresh your memory on the Earth’s annual journey.

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Reason for the Seasons

Folklore tells us that March comes in like a lion and goes out like a lamb. While March is meteorologically a spring month, here in New England and other mid-latitude locations throughout America, it is still cold and snowy when it commences and much milder and rainy at month’s end. Hence the comparison to the ferocity of a lion and the tranquility of a lamb. However, this explanation, attributed to the early American Colonists, is not globally applicable. We must turn to astronomy for a more inclusive answer.

It’s really quite simple. When March begins, the constellation Leo the lion is rising above the eastern horizon just after sunset, and at month’s end Aries the ram can be found setting below the western horizon at sunset. This sky clockwork has not appreciably changed for thousands of years. Our ancestors observed the appearance of these star patterns and ascribed significance to their seasonal arrival and departure in the sky.

Most everyone links the weather with seasonal changes. However, through my decades of public outreach and teaching astronomy, I know many folks are misinformed as to the reason for the season(s). Some individuals believe the northern hemisphere summer is hot because the Earth is closest (perihelion) to the Sun at that point in its orbit. Quite the opposite is true. Earth is at perihelion (approximately 91,000,000 miles) in early January, while farthest (aphelion) from the Sun at the beginning of July (approximately 94,000,000 miles). Even this three million mile difference has little effect on the Earth and its environment. The seasonal changes are the result of the Earth’s axial tilt as it orbits the Sun.

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