June always marks the beginning of summer for us in the Northern Hemisphere. That will happen at 7:09 p.m. Wednesday, June 20. The sun reaches its highest and northernmost point on the celestial sphere at that moment. That is also known as the summer solstice and will give us the longest day and shortest night of the year.

However, this June, a much more rare and dramatic event than the solstice will happen. That will be a transit of Venus across the sun. More than half the Earth will be able to see at least part of this great event, and for a change the northeastern U.S. will actually be able to see some of it.

The most exciting parts of this six-hour event are the beginning and the ending. This time we will be able to see the beginning, called first and second contact; last time, on June 8, 2004, we were able to see the ending, called third and fourth contacts.

This time, first contact will happen at 6:05 p.m. It will take 18 minutes until the other side of Venus also completely enters the surface of the sun. Through a telescope with a safe solar filter, look for the faint and elusive aureole of its illuminated atmosphere just off the sun’s limb. I saw this last time, eight years ago, between third and fourth contacts. It was even more dramatic because I did not expect to see this phenomenon. It showed up as a silvery semicircular arc around the leading edge of Venus, which was actually its thick and poisonous atmosphere illuminated by the sun against the deep blackness of space.

Toward the end of this 18-minute period you could also look for the black drop effect. This is caused by blurring due to our atmosphere and telescopic diffraction, combined with the sun’s strong limb darkening at its edge. You can create a similar effect for yourself by just closing one eye, looking at a bright wall, and barely touching your thumb and forefinger together a few millimeters in front of your eye.

After all this excitement is over, just enjoy watching the planet progress across the sun for about two more hours until the sun sets just after 8 p.m. The whole event takes about 6 1/2 hours. You would need to be at the West Coast to see the whole event and to catch Venus emerging from the sun again.

As you enjoy this rare event — the next one will not occur until December 2117 — reflect on the history and importance of the previous transits and the extreme nature of the planet Venus itself.

Only four pairs of transits have occurred since the telescope was invented more than 400 years ago. No records of any observations of the 1631 transit have been found and only two people are known to have seen and recorded the 1639 transit. However, the 1761 and 1769 transits were a different story since hundreds of astronomers ventured across the globe to carefully observe and record them, hoping to establish the size of the sun and our distance from it.

One of the best navigators of his time, Captain James Cook, sailed to New Zealand and Australia and claimed those countries for England in June of 1769. He was the first navigator to know his position on Earth in both latitude and longitude at all times, which was critical for accurate timings of the transit.

It was easy to determine latitude just by measuring the height of the North Star above your horizon. Longitude was much more difficult to determine because you needed an accurate clock that you could take to sea.

It turned out that an accurate determination of the distance to our sun was not made until after the 1874 transit of Venus. That also established the entire scale of the solar system, because we already knew the ratios of the other planets, thanks to Kepler.

Venus transits always occur in a 243-year pattern that consists of a pair of June transits eight years apart with a gap of 105.5 years and then a pair of December transits eight years apart and then a gap of 121.5 years. If one transit goes across the lower part of the sun, like the one eight years ago did, then the next one in the pair will go across the upper part of the sun. This is a beautiful example of applied mathematics and physics always at work in our solar system.

Venus was once thought to be a paradise, but nothing could be further from the truth. Its surface temperature is a constant of nearly 900 degrees Fahrenheit and its surface pressure is nearly 100 times that of Earth’s. That would be the equivalent of the pressure experienced about 3,000 feet under the ocean. We didn’t get any good images of Venus that could cut through its perpetual clouds until the Magellan mission radar-mapped most of its surface between 1990 and 1994. Many of its secrets were then revealed, but many more mysteries remain for future exploration.

After the transit, we will lose Venus in our evening sky, which will leave only Mars in Leo and Saturn in Virgo. Mercury will take the place of Venus from mid-June to early July. Look for our first planet in the constellation of Gemini low in the west-northwestern sky 45 minutes after sunset, just above and to the right of the slender, waxing crescent moon on the evening of June 21, just after summer starts.

Watch as Mars moves rapidly eastward out of Leo and into Virgo, getting closer to Saturn each evening. Saturn will end its retrograde, or westward motion, on June 26, and remain within 5 degrees of Spica all month. Notice that both orange Mars and golden Saturn are about the same brightness now and that they are both slowly fading.

We may have lost Jupiter and Venus to our evening sky, but they are both rejoining us in the morning sky now. Watch a slender, waning crescent moon drift very close to Jupiter and just above Venus in Taurus 30 minutes before sunrise on the morning of June 17 low in the east-northeastern sky.

June highlights

* June 4. A partial lunar eclipse occurs today, but will not be visible to us in the northeastern U.S. Full moon is at 7:13 a.m.. This is also called the rose or strawberry moon.

* June 5. The transit of Venus starts at 6 p.m. for us.

* June 8. Giovanni Cassini was born on this day in 1625. The large gap in Saturn’s rings is named after this French astronomer; so is the recent mission to Saturn. The last transit of Venus happened in the morning of this day in 2004.

* June 11. The last quarter moon is at 6:42 p.m.

* June 15-17. Jupiter is about 10 degrees above Venus in the morning sky and the waning crescent moon will be joining the pair half an hour before sunrise.

* June 19. New moon is at 11:03 a.m.

* June 26. First quarter moon is at 11:32 p.m. The French astronomer and comet hunter Charles Messier was born on this day in 1730.

* June 30. On this day in 1908, a 100-foot-wide fragment of a comet or asteroid exploded about five miles above Tunguska, Siberia, with the force of 10 megatons of TNT, or about 1,000 times the energy of the Hiroshima atomic bomb. It leveled millions of trees over a 1,000-square-mile area of reindeer country. Even though it didn’t hit the ground directly, shock waves were felt thousands of miles away and suspended dust remained in the atmosphere for months. It is still a mystery, because no craters or meteorites were ever found.

Bernie Reim of Wells is co-director of the Astronomical Society of Northern New England.