On Thursday evening there will be a penumbral and also partial lunar eclipse visible from the UK.

A penumbral lunar eclipse occurs when the Moon travels only through the outer, fainter part of the Earth’s shadow, or ‘penumbra’. This happens when the Earth moves between the Sun and Moon but the three do not form a perfectly straight line. The penumbra causes only a slight darkening of the Moon’s surface, with the Moon still exposed to some direct sunlight, so this type of eclipse is easy to miss.

During the partial phase of the eclipse, part of the Moon travels through the Earth’s full ‘umbral’ shadow. However, on this occasion only a very small section of the Moon will be covered by the umbra at maximum eclipse, though the whole northern half of the Moon will be darkened by the penumbral shadow.

The penumbral phase of the eclipse should start at about 19.03 BST, though it will only be visible after about 20.00 in the UK as before that the Moon will be below the horizon. Roughly speaking, moonrise is earliest toward the east of the UK, latest toward the West (about 20.11 in London and 20.44 in Glasgow on 25 April).

The partial phase of the eclipse begins at 20.54 BST, with greatest eclipse at about 21.07 when the Moon is closest to the centre of the Earth’s full shadow. The Moon will still be near to the horizon, so you’ll need a clear view to the East to see it. The partial eclipse ends at 21.21 after only 27 minutes, making it the second shortest partial lunar eclipse of the century. The penumbral phase ends at 23.11, bringing the whole event to a close.

There will be three lunar eclipses in 2013. However, only two are visible from the UK, the other taking place on 18-19 Oct 2013. Sadly we’ll have to wait till March 2015 for the next Solar eclipse that can be seen from these shores.

19 April 2013 – The annual Lyrid meteor shower is underway and should peak before dawn on Monday 22 April. This year the nearly-full moon is likely to impede viewing, so after midnight from a dark location should give the best chance of seeing shooting stars.

The Lyrids are a reliable annual shower of bright fast meteors associated with Comet Thatcher. The shower gets its name from the constellation Lyra, from which the meteor trails appear to radiate. The shower usually lasts from about 16–26 April.

The Lyrids typically produce between 5-20 meteors per hour at peak, but occasionally the Earth passes through a thicker part of the comet’s dust stream resulting in a more intense shower. In 1982 amateur astronomers counted 90 Lyrid meteors per hour, and in 1803 an even stronger ‘storm’ was observed of up to 700 per hour.

Most Lyrid meteors are around magnitude +2 (roughly similar brightness to the stars in the Plough). A few though, known as ‘Lyrid fireballs’, can be much brighter, casting shadows for an instant and leaving smoky trails of debris.

The Lyrids were first observed 2700 years ago by Chinese astronomers, making them the earliest-known meteor shower.

The next notable annual meteor shower is the Eta Aquarids, a light shower associated with Halley’s Comet and due to peak around 6 May.

27 March 2013 – Did you know that Easter day in the UK is officially an hour shorter this year? Sounds like an April Fool’s hoax but, no, it’s just the start of British Summer Time coinciding with the date of Easter.

This coming Sunday, 31 March, is of course Easter Day (in the Western calendar at least). And this year, as the last Sunday in March it’s also the annual occasion of the clocks going forward one hour. Officially this takes place at 1.00 am GMT (strictly UTC) on Sunday, though of course most people do it on Saturday evening (which means you can still get a full-length Easter day).

The reason why 31 March is the date for clocks to change this year is simply that the Ninth European Parliament and Council Directive on Summer Time Arrangements, currently in force, says so. The Directive states that summer (or daylight saving) time is to be kept between the last Sunday in March to the last Sunday in October, with changes taking place at 01.00 GMT.

But why is 31 March also the date of Easter this year, and why does Easter move around so much?

The simple standard definition of Easter is that it’s the first Sunday after the Full Moon after (or on) the spring equinox. However, this isn’t strictly the case. The vernal equinox used to define Easter is an artificial one always assumed to be on 21 March. The full moon used is also not the true full moon, but an artificial construct based on something called the Metonic cycle, which in turn is thrown out by the insertion of leap years in the Gregorian Calendar.

Again, this may sound like a complicated April Fool’s joke but it really isn’t. To find out more (including a complex algorithm for calculating the date of Easter, valid up to 2099), see our Date of Easter fact file.

And in the meantime, Happy Easter!

7 March 2013 – Here at the Royal Observatory we’re focusing on all things alien at the moment. And this week we have a new visitor in our skies from the furthest reaches of the solar system – the unexcitingly-named C/2011 L4 (aka Comet Pan-STARRS, named for the Hawaiian telescope with which astronomers first spotted it in June 2011).

C/2011 L4 seems to be a fresh comet from the icy and distant Oort Cloud on one of its first visits to the inner solar system. It came closest to Earth a couple of days ago on 5 March, although it was still 100 million miles away, or 1.09 AU (so a little further than the Sun). It’s heading towards the Sun and will reach perihelion (closest approach to the Sun) on 10 March.

This week the comet should be visible very close to the horizon, with best visibility just after dawn. Later in March the comet will be higher in the sky (but fainter) as it heads away from the Sun. Sadly, after this we probably won’t see it again for another 110,000 years, which is thought to be the length of its orbital period around the Sun.

However, as luck would have it another and more spectacular comet, C/2012 S1 (ISON), is due to put in an appearance in November 2013 on its way towards the Sun.