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Image Credit: P Watson-Smith (TAS Observation Officer)
Here Comes The Sun
June is the month when Astronomical Twilight never ends and so the night sky is never really dark. Astronomical Twilight is defined as when the Sun lies between 12 and 18 degrees below the horizon. True night with dark skies occurs only when the Sun is more than 18 degrees below the horizon.
The Summer Solstice occurs on June 22nd, although the longest day is actually on 20th, when we will have over 16 hours of daylight! In terms of the Earth’s orbit, the Solstice marks the point (contrary to all common sense) farthest from the Sun. The Earth is actually closest to the Sun on December 21st, the Winter Solstice and shortest day. What makes it summertime in the Northern Hemisphere is that the Earth’s North Pole is tilted towards the Sun at this time of year. This is what causes the Sun to appear higher in the sky for longer, consequently bathing the Northern Hemisphere in a greater proportion of its heat and light than at other times of the year, giving rise to our Summer. At the Winter Solstice, the opposite is true. The Earth’s North Pole is tilted away from the Sun, the Sun never rises very high into the sky, days are shorter, and we receive a smaller proportion of its heat and light than at other times of the year, giving rise to our Winter.
At the same time the exact opposite occurs in the Southern Hemisphere. Whilst we are enjoying our Summer, the South Pole is tilted away from the Sun and so is experiencing Winter. When we are deep in our Winter, the South Pole is tilted towards the Sun and is enjoying its Summer. Summertime in the Southern Hemisphere occurs when the planet is closest to the Sun AND the South pole is tilted towards the Sun. This is why countries in the Southern Hemisphere have hotter summers than those in the Northern Hemisphere.
No doubt many of you witnessed the Aurora event last month. Normally only seen from places close to the Arctic Circle, only the most active events can be seen from lower latitudes and then often , just as a faint glow on the horizon. So the recent event was exceptional.
The Aurora Borealis, or Northern Lights, occur when there are active areas on the surface of the Sun. These active areas are associated with dark spots known as sunspots. Although apparently dark in images taken with safe solar telescopes, these spots have a surface temperature of around 4000◦C and only appear dark in relation to the solar surface which has a temperature of over 6000◦C. These spots are also responsible for disturbances in the local magnetic field of the Sun, giving rise to the phenomenon of solar flares; huge loops of incandescent material that shoot out from the surface and into space. Whilst most of this material is held by the magnetic field of the Sun and falls back to the surface, particularly large spots, or groups of very active spots, can produce flares from which the material escapes the magnetic and gravitational field of the Sun. This material, travelling through interplanetary space is known as the “solar wind”. It is actually a stream of ionized particles ejected from the Sun’s surface. When this stream interacts with the Earth’s own magnetic field these particles are funnelled towards the “weak” points at the North and South Poles. These ionized particles then interact with gases in the atmosphere to produce the stunning colours seen in the Aurora Borealis and its counterpart the Aurora Australis.
The activity of the Sun, and thence the Aurorae, rises and falls in an 11-year cycle. Historic observations of sunspots have shown that the number of spots rises from a minimum to a maximum over this period. This cycle affects our weather. Most notably, a particularly long period, known as the Maunder Minimum, where almost no spots were observed for a number of years, coincided with the period of very cold winters, freezing rivers and Frost Fairs experienced in Northern Europe during the Victorian era, and which we can read about in the works of Charles Dickens and other contemporary authors. Conversely active Maxima have coincided with years of long, hot summers.
The next predicted maximum is set to occur in May/June 2025. So last month’s Northern Lights may herald a season of such displays, as the Sun is currently very active and shows a greater number of spots. With luck, there may be more to come!
As a caveat, extreme events, known as Coronal Mass Ejections, can produce enough electromagnetic radiation to wipe out all electronic systems in its path. If such a CME was to head straight for the Earth there is little we could do about it. Such events have been recorded in the past, fortunately before the advent of the widespread use of electricity, although Northern Canada experienced a widespread power blackout after a geomagnetic storm in 1989.
Sky Notes
Please note all times given in this article are in GMT so remember to add an hour to get the time in BST.
See June Sky Schedule
Sun: The Sun is never very far below the horizon at night throughout June and into July, so it never gets astronomically dark, and this is not the best time for observation of faint fuzzy objects. However stars and planets can still be observed particularly around midnight when the sky is at its darkest. The Milky Way is visible from dark sites stretching across the Northern horizon.
Mercury and Venus: Both of these inner planets are closer to the Sun than the Earth so are never very far from the Sun, especially Mercury, which may possibly be glimpsed low in the North-East shortly after sunset at the end of the month. Venus is too close to the Sun to be seen this month.
Mars: Mars rises about 1.30am on the eastern horizon. During the month, the red planet moves from the background stars of Pisces into those of Aries.
Jupiter: Rises in the North-East at around 2.30am and can be spotted until dawn (before 4:00am at this time of year). Jupiter is bright enough to be easily spotted if you have a low enough horizon. The giant planet lies in the constellation of Taurus.
Saturn: The ringed planet can also be found rising in the East around midnight but is fainter than Jupiter because it is further away from the Earth. At this time, the ringed planet lies in the constellation of Aquarius.
Uranus and Neptune: Uranus is not visible this month. Neptune rises half-an-hour after midnight and lies in the constellation of Pisces.
Meteor Shower: There are no major meteor showers this month, but you could still see stray (sporadic) meteors and meteors from a couple of minor showers; the Arietid shower from 14th – 24th occurs during the daytime and the Bootid shower which peaks on 27th. Neither are strong showers and produce few meteors.
Noctilucent Clouds: It may seem anachronistic for astronomers to be interested in clouds, but these rare apparitions are best seen at this time of year. Noctilucent clouds are so high (over 80 km) in the atmosphere that they are able to reflect sunlight even when the Sun is well below the horizon. The tiny ice crystals that make up the clouds reflect bluish-white light that stands out against the gloaming sky. They are typically seen 1 ½ to 2 hours after sunset low in the Northwest or before sunrise low in the Northeast.
Comet: There are no bright comets predicted this month although a faint periodic comet known as 13P/Olbers makes its closest approach to the Sun on June 30th.
The New Moon occurs on June 6th with First Quarter on June 14th. Full Moon follows on 22nd, the night of the Summer Solstice, and Last Quarter at the end of the month on June 28th.
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