The Night Sky in April 2006

Looking at the night sky with the naked eye or using a small telescope can be fascinating, so each month these notes keep you up-to-date with unfolding events.

Maps of the current night sky can be seen by clicking HERE

Phases of the Moon

New Moon Thursday 27	First quarter Wednesday 5	Full Moon Thursday 13	Last quarter Friday 21

Click HERE to see the phase of the Moon on each day of the month.

 

Monthly Highlights

Although we had a bit of a cold spell during March, the worst of it seems to be behind us as we move slowly towards the summer. With the clocks changing at the end of last month, it now stays light until later in the evening, and it's approaching 9 pm before the sky is dark enough to look at the stars and planets. If you do get to stay up later than that, then you may be able to spot one of the following monthly highlights:

April 9th and 25th - Apsides of the Moon

Like most orbits in our Solar System the Moon's orbit is eccentric, i.e. the Moon's distance from Earth varies during the lunar month. Because of this change in distance, the Moon appears to vary slightly in size, although most observers would find it hard to spot any difference. The term used to describe when the Moon is closest to the Earth is perigee, which this month occurs on the 25th. The Moon is farthest from Earth on the 9th April, a point known as apogee. These two terms are known as the Apsides of the Moon.

The above image shows how the size of the Moon varies during the month. At perigee (closest), the apparent size of the Moon is around 33 minutes of arc in diameter, whereas at apogee (farthest), the apparent size of the Moon is around 29 minutes of arc. Remember that there are 60 minutes in one degree of arc. It is no surprise that the tides are stronger when the moon is closer (perigee), and it is the combination of the Moon being at perigee and also lining up with the Sun that leads to unusually high spring tides known as proxigean spring tides, which occur once every 1.5 years.

If you want to learn more about tides, click on the following link.

The Moon's orbit is expanding over time as it slows down (the Earth is also slowing down as it loses energy). For example, a billion years ago, the Moon was much closer to the Earth (roughly 200,000 kms) and took only 20 days to orbit the Earth. Also, one Earth 'day' was about 18 hours long (instead of our 24 hour day).

The Planets

Click HERE and start the Electric Orrery to see where all the planets are today in relation to each other.

Mercury Mercury starts the month as a steady morning 'star', rising just 30 minutes before the Sun. It will reach maximum elongation (or separation) from the Sun on the 8th, when it will be an angle of 28 degrees to the West. It will spend the rest of April as a morning object as it positions itself to move around behind the Sun during May. Unfortunately, because Mercury is always low when the Sun is about, it is one of those objects that the Liverpool telescope cannot safely observe. Venus Venus continues to overtake us on the inside track and will remain a morning object and brightest planet for most of 2006, finally passing behind the Sun in late October. Having just passed greatest elongation (47 degrees West of the Sun) in late March, early birds can spot Venus around 5.10 am, a good 80 minutes before sunrise. During the month this will decrease by 10 minutes as the planet's separation reduces. Why not use the electric orrery to see how Venus remains a morning object over the next few months. Mars Mars remains visible throughout the month but continues to diminish in size and brightness as Earth moves away. It can be found high above the south-western horizon shortly after sunset and spends half the night crossing the sky until it sets in the northwest around 1am. This moves closer to midnight by the end of the month as we continue to move away. Now only slightly brighter than stars around it, Mars will be noticeable only by its slightly red hue and the fact that it moves from night to night. We can still observe Mars using the Liverpool telescope, but sadly it will be another 18 months before we get really detailed images again. Jupiter Jupiter is now the best target to observe as we continue to catch it up on our inside orbit or track. It will increase in both size and brightness until it reaches opposition (other side of Earth to the Sun) at the beginning of May. At the start of April it can be found rising in the southeast around 10.30 pm, but by sunrise it will have made its way to a position low above the western horizon. Jupiter remains the second brightest planet after Venus. Why not use Go Observing to obtain an image of Jupiter and see if you can capture its elusive red spot, which is on show at the moment. Saturn Saturn continues to decrease in size and brightness as we move slowly away. At the beginning of April, Saturn will be found high to the south-southeast shortly after sunset and spends the night crossing the sky before setting in the west-northwest around 4.25 in the morning. This moves back to 3 am by the end of the month. If you act fast in requesting an observation, you could still bag a decent picture of Saturn, although your chances are growing less as the month moves on.   Constellation of the month Each month, new constellations are highlighted for you to identify in the night sky. This month we will be looking for the winter constellation of Hydra, the Snake. Hydra is the largest of the modern constellations, covering nearly 3% of the night sky. Despite its size, it only contains one bright star, Alphard, so can be difficult to spot unless you have really dark skies. Hydra is a long constellation that can be found stretching low above the southern horizon close to midnight. The constellation of Corvus, which looks like a cup, will be a good indicator to the location of Hydra, but you can also use Jupiter and Saturn to guide you to the ends of the snake. Good hunting! April Night Sky Hydra contains an interesting object known as a Mira Variable star. Mira variables are a kind of pulsating variable star that have very red colours, pulsation periods longer than 100 days, and tend to increase in brightness by more than 2.5 times. They are red giant stars in the very late stages of their lifetime, which will soon expel their outer layers as planetary nebulae and become white dwarfs within a few million years. Animation of Mira Variable Animation of a Mira Variable  ©NASA The brightness of R Hydrae varies over a period of 389 days by nearly 1500 times. At maximum brightness the star can be seen with the naked eye, while at minimum a telescope of at least 5 cm is needed. Mythology: Hydra The constellation Hydra resembles a twisting snake, and features as such in some ancient myths. In Greek mythology, a crow was tasked by Apollo to fetch water but decided to rest lazily during the journey. After finally obtaining the water in a cup, it returned with a water snake as an excuse for the delay. According to the myth, Apollo saw through the fraud, and angrily cast the crow, cup, and snake into the sky where they remain to this day as the constellations of Crater, Corvus and Hydra.