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Monthly Skywatchers' Page For London and the UK The Sun and eight major planets (plus KBO Pluto) to scale. Earth is third planet from the left.

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Introduction

The planets resemble stars except that, generally, they do not twinkle. Unlike the stars whose patterns are fixed, planets wander through the sky changing their positions amongst the starry background. This means that their periods of visibility change as the relative position of the Earth, Sun and planet vary. In one year Mars may be visible in August at midnight. In another year it may be behind the Sun and invisible from Earth during August.

This page gives the details of visibility for the five naked eye planets: Mercury, Venus, Mars, Jupiter and Saturn. It also gives information about comets, meteor showers and non-regular phenomena like eclipses, transits and occultations.

The Observers' Glossary explains the terms used. Alternatively run the mouse cursor over terms in maroon.

July 2009

Mercury

Venus

Look for Venus close to the old crescent Moon on the mornings of 18th and 19th.

Mars

Look for Mars close to and below the old cresecent Moon on 18th.

Jupiter

Look for the slightly yellowish planet low in the South East during the evening and in the South after midnight. Jupiter is the second brightest planet (after Venus) so should be obvious once it rises as it is brighter than any star.

The Moon will be close to the planet on 10th. The distant planet Neptune can be found below Jupiter on 13th using a telescope and a good map.

Saturn

The Sun

At aphelion, the Sun's distance from the Earth is 152.5 million km (as compared to 147.5 million km in early January). This is a difference of some 5 million km. The further Sun gives the Northern Hemisphere summer 7% less heat than it would otherwise receive from the Sun. The Sun's distance from the Earth does not cause the seasons. These are caused by the Earth's axis being tilted at an angle of 23.5° to the plane of its orbit around the Sun. It is this that causes both the length of the day and the noon day altitude of the Sun to vary throughout the year. It also causes the seasons to be reversed in the Southern Hemisphere.

The Sun enters Cancer on 20th. This date does not tie in with astrology as astrologers are using dates from 2000 years ago.

The Moon

Solar Eclipse

Eclipses of the Sun occur when the Moon - in its monthly orbit around the Earth - passes between the Earth and the Sun. This can only happen at New Moon. The Moon's shadow (or umbra) passes across the Earth and any observer that finds themselves within it will experience a Total Eclipse of the Sun. The umbra is quite narrow, rarely more than a couple of hundred kilometres wide. Observers in the much wider penumbra will see a partial eclipse. Outside of the umbra or penumbra, no eclipse is visible.

During a Total Eclipse, the Sun is slowly covered by the Moon until it is completely hidden behind the Moon. The covering takes between an hour and a hour and a half and is called the Partial Phase. Looking at the Sun during a partial eclipse can only be done with special glasses - it is then possible to see the "bite" taken out of the Sun by the encroaching Moon.

The early part of the partial eclipse is not noticeable in terms of changes to the surroundings. Towards the end of the partial phase, the light takes on a strange quality and the sky turns an intense blue. Winds might change and the temperature may fall significantly. Animals and birds begin to behave strangely, the latter flying around looking for a place to roost. Venus may become visible as a white dot in the sky.

When the Sun is completely covered, Totality begins and the sky darkens rapidly. The light fades from day to a very late twilight in just a few seconds, an eerie effect that disorientates those experiencing it for the first time.

During totality, the sky becomes dark enough for stars and planets to be visible. Filters can be put down as the "Sun" can now be viewed with the naked eye. Along the horizon, the red colour of sunset is visible all around. Bird song or insect chirping can often be heard.

Looking up at the Sun, the Sun's magnificent CORONA is now visible, a pearly white delicate structure often marked with lines of the Sun's magnetic field. Sometimes red flames (called PROMINANCES) can be seen next to the Moon's jet black body - these are explosions of the Sun's surface which are not normally visible.

Totality is one of nature's great natural phenomena but is over all too quickly, lasting only a few minutes. But oh, what a few minutes...

When Totality ends, the sky brightens rapidly. Often the first piece of sunlight to return will pass through a lunar valley as the Moon is quite a rough body - not smooth like a billiard ball. This point of light is called the Diamond Ring and is normally very spectacular. The eclipse continues with another partial phase as the Moon moves away from the Sun. Finally the eclipse is over.

The umbra is about 250 kilometres wide at its widest. At any location along the path of totality, the best view will be along or close to the centre line of that path.

The region outside the umbra (shown with light blue lines) is the much larger area where only a partial eclipse will be visible. Notice how relatively narrow the path of totality is.

The umbra then crosses the River Ganges and passes the Hindu holy city of Varanasi (3m 01s) and Patna (3m 44s). There will be many a Hindu prayer performed that morning. The shadow then crosses into Eastern India passing briefly into Nepal, Bhutan and Bangladesh. The North East Indian town of Dibrugarh close to the Himalaya will see 3m 19s of totality.

The umbra clips Burma before entering China where it will remain for the next 40 minutes or so. At the India-China border the length of totality has increased to 4m 26s. Most of remote and mountainous Sichuan Province is covered by the shadow. The local capital, Chengdu, experiences totality for 3m 16s. On the central line of the path (88km from the city) totality lasts for 4m 52s.

The industrial and hilly city of Chongqing will be totally eclipsed for 4m 06s even though it is 70km away from the centre line of the path. As the shadow moves East, it passes along the Yangtze River, more or less following the world's third longest river. Wuhan, a large port city on the river (and China's fourth largest metropolis) is just 20km south of the centre line - it will be plunged into darkness for 5m 25s. It will now be mid-morning with the Sun fairly high in the sky. Hangzhou, a lakeside city, will be eclipsed for 5m 19s.

Shanghai (China's largest city with a population of about 19 million) is 66km north of the centre line and will be totally eclipsed for 5m 00s. The beach resort of Jinshanwei, only 10km from the centre line and about 70km from Shanghai, will receive 5m 54s of darkness during late morning, around 9:30 local time. The umbra will be 249km wide and travelling at a velocity of 830 m/s. I am hoping to be here with clear skies.

The umbra then leaves China and heads out to sea passing several Chinese islands in the bay.

The umbra passes across the Pacific Ocean crossing a group of Japanese islands which are difficult to get to. The largest island, Yakushima, will be eclipsed for 3m 57s as it is quite far from the centre line. The closest island to the centre line is Akuseki-shima and this will be eclipsed for 6m 20s. About 25 minutes later, the remote Japanese island of Kitaio Jima will be darkened for 6m 24s.

The umbra will have travelled a distance of 15,150 km having taken 3 hours and 25 minutes to cross from one side of the Earth to the other. The shadow will have covered just 0.71% of the surface of the Earth, most of that ocean. The small amount of land covered by the umbra during a total eclipse explains why they are so rare for any one place.

Total eclipses of the Sun occur roughly seven or eight times every decade but the area they cover is minute compared to the size of the Earth. On average, they happen in one location once every 400 years or so. For example, in London: the last time totality occurred in the UK capital was on 3 May 1715. The next totality in the city will be on 14 June 2151. This is a gap of 436 years. This is why most people never see a Total Eclipse of the Sun. To experience this phenomenon it is necessary to travel, unless you are very lucky.

The total eclipse of 22 July 2009 is the longest of the 21st century (maximum duration 6m 39s). It is the longest total eclipse since 11 July 1991 (maximum duration 6m 52s off the coast of Mexico). The next total eclipse that will have a longer duration will occur on 13 June 2132 and will last for 6m 55s.

1. Less than five hours before the eclipse begins, the Moon in its monthly orbit around the Earth, is at its closest. This makes it appear larger than average in the sky.

2. On 4 July the Sun is at its furthest from the Earth. The distant July Sun appears smaller than average.

3. The alignment between the Earth - Moon - Sun is almost a straight line, just 7% out. The Moon's umbra falls straight down onto the central regions of the Earth (rather than at high latitudes where its hits the Earth at an angle). Observers in the tropics are closer to the Moon than if they were near the polar regions. Also, the Earth rotates faster in the tropics carrying the observer along with the shadow so their relative speeds are lower. It takes the shadow longer to pass the rapidly moving tropical observer.

I will be more than happy with 5m 54s in China. If it is clear this will be my second longest totality and my 12th total eclipse.

On 28 May 585 BC, Cyaxares of Media (modern Iran) and Alyattes of Lydia (modern Turkey) were about to start a battle when a total eclipse of the Sun occurred. The two nations were so frightened that they signed a peace treaty. Because eclipses can be predicted very accurately by astronomers, this remains the earliest historical event that can be dated to the exact day.

Next Month

All times on this page are London (UK) times.

Sources: Astronomy Now magazine, Starry Night Pro, USA Naval Observatory and UK Nautical Almanac Office.

© 2009 KryssTal
All sky images by Starry Night Pro

Books From Amazon.co.uk

Observers' Glossary

The Observers' Glossary

KryssTal Related Pages

The Solar System

Measuring The Stars

The Brightest Stars

Eclipses, Transits and Occultations

The Scale of the Universe

A Brief History of Astronomy

The Calendar

Gravity

Spherical Trigonometry

External Skywatching Links

StarDate Online
An excellent online skywatchers' magazine featuring detailed monthly information about astronomical events.

Astronomy Picture of the Day
A different image each day with an extensive archive and explanations.

Society for Popular Astronomy
Monthly planet and star gazing details with maps for young people.

Telescope House
A London based source of astronomical equipment, telescopes, software, maps and books.

Southern Sky Watch
Sky watchers information for the southern hemisphere.

Starry Night Pro
Superb planetarium software. Simulate the sky from anywhere on the Earth and any time.