|Events during June
· The Summer Solstice, when the Sun reaches its most northerly point over the Tropic of Cancer, is on the 20th.
· The Moon: New – 3rd, Full – 18th.
||Mercury passes through inferior conjunction on the 7th June, then moves away from the Sun to reach
greatest western elongation (22º) on July 1st. In the northern hemisphere the long summer twilight hampers visibility of the planet.
The best times to observe Mercury are when it is an evening star in the spring and a morning star in the autumn. In midsummer
the lighter skies make visibility difficult near the horizon.
||Venus passes through superior conjunction on the 9th June and is therefore too close to the
Sun for observation. For a while the planet is occulted by the Sun and therefore undetectable by any Earth-based instrument. This conjunction
is exactly halfway between the two transits of Venus that occur this century in 2004 and 2012.
On the 8th June 2004, Venus was at inferior conjunction
and transited the sun. Transits of Venus are rare, taking place at greater
than 100 year intervals and usually in pairs. The last two transits of Venus were in 1874 and 1882. June's transit
began at 7.20h and lasted 6 hours until 13.20h, the total event visible from Europe as a small black disc crossing
the lower part of the Sun from left to right. The next transit will be in late June 2012. After that, transits of Venus
won't occur again until 2117 and 2125.
Before and after inferior conjuction, when Venus is
the closest it comes to the Earth, are the times at which the planet is most brilliant and can be seen setting or rising
4 hours after or before the Sun. The dates of the next two inferior conjunctions are October 28th 2010 and October 26th 2018.
||Mars fades further to +1.6 magnitude and is visible in the evening western sky. Towards the end of June,
Mars moves from Cancer into Leo and closes in on Saturn within 5º by the end of the month, when it also passes 0.8º north of Regulus.
At opposition on the 28th August 2003, Mars was only 56 million kilometres from the Earth. It showed a
disc of 25.1 seconds of arc across which is almost as large as it can ever appear. Mars started 2003 at 310 million kilometres from
the earth at 4.5 seconds of arc and 1.6 magnitude. By opposition it brightened 50 times to reach -2.9 magnitude but faded to 0
magnitude by December. Even to the naked eye Mars was a striking object in the summer and autumn sky, easily identifiable by its
reddish hue in an area of sky poor in bright stars. Mars will not be as close again until 2018.
These favourable oppositions occur every 15 or 17 years but other oppositions occur at average intervals of 2 years 2 months during
which time the planet makes a complete circle of the Earth. In general Mars is observable every other year, being too close to the sun for favourable conditions
during other times. Brightness at opposition varies from -1.0 to -2.9 magnitude, and when furthest from the earth it fades to 1.7 magnitude. The planet can be
identified by its orange-red colour.
As in 2003, Mars comes nearest to the Earth at oppositions at the end of August. At these times it can be brighter than Jupiter,
although low in the sky in Aquarius for northern observors. In the northern hemisphere, the planet may be better seen at oppostions during autumn and winter months
when it is higher in the sky.
||Jupiter brightens further to a brilliant -2.7 magnitude as it approaches opposition early next month.
Continuing in a retrograde motion since its stationary point last month on May 9th, it is visible in the south-eastern sky before midnight.
Being 770 million kilometres from the sun, the difference in brightness between opposition and conjunction varies less than with Mars, from
about -2.8 to -1.8 magnitude. Always a bright planet, Jupiter comes to opposition a month later each year, moving approximately from one zodiacal constellation to the next.
The 4 largest moons of Jupiter are easily visible through a small telescope, ranging from 4.6 to 5.6 in magnitude. The innermost, Io, takes 1.8 days
to orbit the planet making its motion easily detectable within a few minutes.
||Saturn is at +0.9 magnitude, visible in the evening western sky. It is noticeably brighter than Mars which
is moving eastwards towards it and will overtake next month.
Saturn moves more slowly than Jupiter and can remain in the same constellation for several years. The brightness of the planet depends on the
aspect of its rings, as well as its distance from Earth and the Sun.
The planet crossed the equator into the northern hemisphere in 1996 where it will remain until 2010 with the southern side of the ring system
facing the earth. Because of its distance, its brightness varies little between opposition and conjunction but is affected by the huge ring system. Seen edge on the
rings contribute little or no light.
Every 15 years the plane of Saturn's rings passes through the sun, illuminating first the north and then the south side. For a few days the
rings are edge on to the sun. About the same time the Earth passes through the ring plane and, depending on the Earth's position, this may happen just once or 3 times.
During 1995/96 there was a triple crossing and the next will be 2038/39. The next single crossings will be in 2009 and 2025.
Saturn's largest moon, Titan, is visible in small telescopes orbiting outside of the ring system.
||Brightness varies slightly reaching 5.6 magnitude at opposition. This is bright enough to see with the naked eye but identifying it against the stars is difficult.
||Neptune has an average magnitude of 7.9 which varies little with changing distance.
||Pluto reaches opposition on the 20th June in Sagittarius at a distance of 4,558 million kilometres and a magnitude of +14.
No longer an offical planet and never brighter than +13 magnitude, Pluto is only visible through powerful
New moon: 3rd
First quarter: 10th
Full moon: 18th
Last quarter: 26th