|Events during April
· 8th - Mercury is at greatest elongaion (19º).
· 28th - Mercury is at inferior conjunction.
· The Lyrids meteor shower continues from the 19th to 25th April with maximum meteors on the 21st and 22nd. Conditions for viewing are
favourable this year.
· The Moon: New - 14th, Full - 28th.
Apogee (405,000 kilometres) - 9th, Perigee (367,140 kilometres) - 24th.
· If you are lucky enough to be near the equator on an April evening, you will be able to see the best known northern constellation,
Ursa Major, and the best known southern constellation,
the Southern Cross, above the horizon at the same time.
Mercury reaches greatest easten elongation (19º) on April 8th and is
visible in the western evening sky for the first ten days of the month for observors in tropical and northern latitudes. For observors
in the northern hemisphere, this is the most favourable evening apparition of the year.
During this period of visibility, which began in late March, Mercury fades from -1.4 to +1.0 magnitude. The brilliant Venus will be in the same part
of the sky as Mercury and is a useful guide to locating the fainter planet. In Europe and north America, between the 3rd and 8th April, at the end of
evening twilight Venus and Mercury will be at the same altitude above the horizon with Venus slightly further to the west and 25 times brighter than Mercury.
The best times to observe Mercury in the northen hemisphere 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.
Still at magnitude -3.9, Venus is a brilliant object in the western evening sky after sunset.
See Mercury above for the proximity of the two planets this month. On the evening of the 16th April, Venus is below the two-day old waxing crescent Moon. By
the end of April, in northern latitudes, Venus sets more than two and a half hours after the Sun.
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, moving directly in Cancer
can be seen immediately after dark but fades from +0.2 to +0.7 magnitude during April.
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, in Aquarius, is visible to observors
in equatorial and southern latitudes in the eastern twilight sky before dawn.
Varying from 603 (at its closest) to 770 million kilometres from the sun, the difference in brightness between opposition and conjunction varies less than
with Mars, from about -2.9 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 remains in Virgo, decreasing slightly
from +0.6 to +0.8 magnitude, moving retrograde. The ring angle decreases slightly to just 2º by the
end of April.
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 a maximum of +5.6 magnitude at opposition. This is bright
enough to see with the naked eye but identifying it against the stars is difficult. At closest approach, Uranus is 2,856 million kilometres (1,775 million miles)
Neptune has an average magnitude of 7.9 which varies little with changing distance. At its closest,
Neptune is 4,341 million kilometres (2,697 million miles) from Earth.
No longer an offical planet and never brighter than +13 magnitude, Pluto is only visible through powerful telescopes.
Last quarter: 6th
New moon: 14th
First quarter: 21st
Full moon: 28th