In 2020, the summer solstice will take place on June 20 at 5:44 p.m. EDT, which means the night of June 20-21 will be the shortest of the year. This solstice usually occurs on June 21, but it can fall anywhere between the 20th (like this year) and the 22nd. Let’s explore this curious shifting of dates in more detail.
Solstices and equinoxes
Night and day are the result of the Earth spinning on its axis; imagine this axis of rotation as a rod that runs through the Earth from pole to pole. Our planet’s orbit around the Sun defines a plane in space. Instead of being perpendicular to this plane, Earth’s rotational axis is slightly tilted at an angle of about 23 degrees. In fact, this inclination of our planet’s poles is the reason we have seasons, equinoxes, and solstices. If the Earth’s axis were perpendicular to its orbital plane, the seasons as we know them would not exist, except for slight annual changes in temperatures caused by the varying distance between the Earth and the Sun.
In our hemisphere, when the North Pole is tilted closest to the Sun, the days are longer and the Sun climbs higher in the sky. Conversely, when the South Pole is pointed toward the Sun, our days are shorter and the Sun stays low on the horizon. The solstices are the moments when one of the poles is tilted as close to the Sun as it will ever be during the year. The equinoxes occur midway between the solstices, when the Sun crosses directly over the Earth’s equator. These moments mark the beginning of the astronomical seasons.
While the June solstice signals the arrival of warm summer days for us, it also significantly narrows our celestial viewing opportunities: At our latitude, Montreal will have 15 hours and 38 minutes of sunshine on that day, and a mere four hours of truly dark skies between dusk and dawn. The Sun will peak at 67 degrees above the horizon at solar noon.
Tropical year and time zones
It takes Earth an average of 365 days, 5 hours, 48 minutes and 46 seconds to return to the summer solstice position; this is called a tropical year. Yet June 21 comes around every 365 days. Therefore, each year, the solstice occurs 5 hours 48 minutes and 46 seconds later than it did the previous year. To ensure the solstices and equinoxes fall around the same dates and to keep our calendars synchronized with the seasons, we add an extra day—February 29—to the calendar every four years, known as a leap year.
Since 2020 is a leap year, the June 21 date is pushed back by 24 hours compared to last year. The summer solstice, which occurred on the 21st in 2019, will therefore take place on the 20th this year. For Quebecers, the next June 21 solstice will not happen again until 2022.
There’s another factor to consider in these shifting dates. The summer solstice will occur at exactly 5:44 p.m. here in Quebec, but what about in the rest of the world? Solstice is the moment when Earth reaches a specific point in its orbit, and this moment occurs at a different time in each time zone. In Moscow, for example, this year’s summer solstice takes place on June 21 at 12:44 a.m. local time; in Tokyo, it’ll be at 6:44 a.m., also on June 21.
The Moon and the late planets
June opens up with a waxing gibbous Moon that turns full on the 5th as it hangs in close proximity to the galactic centre. The Moon’s first two planetary visits take place a few days later when it cozies up to Jupiter on the night of June 7-8 and then Saturn on the following night. This, of course, means that the two giants currently appear close together in the sky, but they’re due to get even closer in the coming months. Jupiter and Saturn rise in the southeast just before midnight in early June, then increasingly earlier as the weeks go by, but the pair will remain dominant at the end of the night and at dawn. Look for them straddling the border of Capricorn and Sagittarius.
Mars and the occultation of Venus
The last quarter Moon will pass a few degrees below Mars on the morning of June 13. The Red Planet rises around 2 a.m. in early June and a little after 12:30 a.m. by month’s end. However, the biggest celestial event of the month happens when Venus will be occulted by the Moon on the morning of June 19, just before sunrise. Unfortunately for Montrealers, first contact will occur just below the horizon, but Venus’s reappearance from behind our satellite will be visible at 4:11 a.m., very low on the east-northeastern horizon. The Moon will be new on the day after the solstice, and the first quarter Moon will occur on June 28.
Bonnes observations !