Beginning of Summer 2021

Home , Summer solstice

Table of Contents

The beginning of summer 2

Eclipses and related phenomena 3

Night sky observation in summer 4

Astronomical Almanac 4

______© Instituto Geográfico Nacional c/ General Ibáñez Ibero 3. 28003 - Madrid – España 1

1. The beginning of summer

The summer of 2021 will begin on June 21 at 5:32 official Madrid time, according to calculations by the Observatorio Astronómico Nacional (Instituto Geográfico Nacional - Ministerio de Transportes, Movilidad y Agenda Urbana). This season will last 93 days and 15 hours, and will end on September 22 with the beginning of autumn.

Jupiter and Saturn will be visible at dawn at the beginning of the summer, while they will be visible almost the whole night during the month of August, and will finish the season being visible only at dusk. At dusk Venus will also be visible during the whole season, while Mars will be visible at dusk until the end of August.

The beginning of the seasons is set, by definition, by the moment when the Earth is in certain positions in its orbit around the Sun. In the case of summer, this position takes place when the Sun reaches its northernmost point along the ecliptic. The day when this happens, the Sun reaches its maximum northern declination (+23º 27') and for several days it remains with its maximum height at noon almost unchanged. This circumstance is also called the summer solstice ("still sun"). At this moment, winter begins in the southern hemisphere.

The day of the summer solstice corresponds to the largest number of daylight hours of the year. Around this date, the Sun rises earliest and sets latest. Even though unrelated to the beginning of the seasons, another phenomenon that occurs at this time of the year is the aphelion, that is, the day when the Sun and the Earth are farthest apart from each other in the whole year. It is this greater distance from the Sun that causes the Earth to move more slowly along its elliptical orbit during the summer (according to Kepler's second law) and therefore this season is the longest.

Possible dates for the beginning of summer. The beginning of summer can occur, at most, in three different dates of the calendar (from June 20 to 22). Throughout the 21st century, summer will begin on June 20 and 21 (official Spanish date), with the earliest start being in 2096 and the latest start taking place in 2003. The variations from one year to another are due to the way the sequence of calendar years (some leap years, others not) fits with the duration of the orbit of the Earth around the Sun (a period known as tropical year).

Duration of the day. If we colloquially call the length of the day the time that passes between sunrise and sunset in a given place, June 21 will be the longest day of 2021. As an example, in Madrid this day will last for 15 hours and 3 minutes, compared to 9 hours and 17 minutes on the shortest day (which in 2021 will be December 21). Note that there is almost a six hours difference between the shortest and longest day. This difference depends a lot on the latitude of the location, being zero at the equator and being maximum (24 hours) between the polar circles and the poles. It is precisely above the northern polar circle where some days of the year around June 21 the midnight sun phenomenon takes place, in which the Sun is visible above the horizon during 24 hours of the day.

______© Instituto Geográfico Nacional c/ General Ibáñez Ibero 3. 28003 - Madrid – España 2

Sunrise and sunset. One might think that the longest day of the year will also be the day when the Sun rises earlier and sets later; but this is not so: this is because the Earth's orbit around the Sun is not circular but elliptical, and because the Earth's axis is tilted in a direction that is different from the axis of this ellipse. This also makes a sundial and our clocks, based on a fictitious Sun, disadjusted. The earliest sunrise will take place on June 14, while the day of the latest sunset will be June 27.

Distance to the Sun. Around this time of the year, the maximum annual separation (aphelion) between the Earth and the Sun takes place. This time, the maximum separation will take place on July 5, with a distance of just over 152 million km, about 5 million km more than at the time of perihelion or shortest distance (January 2, 2021).

Solar activity. The Sun's activity is characterized by the presence of sunspots, flares and prominences on its surface, and from Earth, it can affect the propagation of radio waves and results in a greater presence of polar auroras. This solar activity follows a period of approximately 11 years, and is associated with the Sun's magnetic cycle. We are currently in the 25th solar cycle which began in December 2019 and is predicted to peak in July 2025. According to estimates made by NOAA and the Space Weather Prediction Center, during summer the number of sunspots will reach values between 27 and 53. Plots with the number of sunspots in recent years and predictions of the evolution for the 25th cycle can be found at: http://www.swpc.noaa.gov/products/solar-cycle-progression

2. Eclipses and related phenomena

During the summer, no eclipses will take place.

3. Night sky observation in summer

At any time of the year there are astronomical phenomena of interest, either predicted (such as eclipses) or not (such as new comets). It is usually preferable to carry out the observations on dates close to the New Moon (July 10, August 8, September 7), except when we aim to observe the Moon itself.

Full Moon. The first Full Moon of summer will take place on June 24, while the subsequent Full Moons will be spaced by periods of 29 or 30 days. This summer, three Full Moons will take place: July 24, August 22 and September 21.

Visibility of the planets. At the beginning of the summer, Jupiter and Saturn will be visible at dawn; later on, they will be visible almost the whole night during part of the month of August; finally, they will end the season being visible only at dusk. At dusk, Venus will be visible and, until the end of August, also Mars.

Meteor showers. Without a telescope, occasional meteor showers can be observed. The two most important meteor showers of summer are the delta Aquarids, with a maximum around July 30, and the popular Perseids, whose maximum takes place around August 12. More information at: https://astronomia.ign.es/web/guest/lluvias-de-meteoros

Constellations. As for the fictitious groups of stars known as constellations, possibly summer is the time of year when most people look up to the sky, and the focus of attention is the spectacular whitish band that forms the Milky Way. As soon as the night falls, the brightest stars begin to appear and the triangle formed by Altair in Aquila, Deneb in Cygnus and Vega in Lyra is easily identifiable. It is the so-called summer triangle. Around the North Star, the constellations of Cassiopeia, Cepheus, Cygnus, Draco and the two Bears will be seen throughout the night. Also visible from East to West will be Pegasus, Aquila, the Corona Borealis and Coma Berenice. Near the horizon we will be able to observe some of the zodiacal constellations throughout the night, from Virgo to Aquarius, the latter already almost at dawn.

Observations with binoculars or small telescopes. With large binoculars or a small telescope, equipped with an appropriate lunar filter, the relief of the Moon can be observed. In order to have a good insight, it is recommendable to observe the Moon night after night while the phase increases, because with this strategy new orographic accidents will be perceived each day. When the night is darker because of the New Moon, you can try to see emission nebulae such as the Orion nebula complex (Messier 42 and 43), the Pleiades group of stars and the supernova remnant known as the Crab nebula (Messier 1). With binoculars you can also see the brightest moons of Jupiter and take a tour of the starry band that makes up the Milky Way.

4. Astronomical Almanac

For more information on the astronomical phenomena of the year, you can consult the book published yearly by the Instituto Geográfico Nacional (Anuario Astronómico).

Information provided by the Observatorio Astronómico Nacional (IGN, Ministerio de Transportes, Movilidad y Agenda Urbana). Reproduction authorized provided that the source is acknowledged.