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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 location of the stars and can be learnt throughout a single year from a book. Each month of the year, the same stars are visible from a given location. Different stars and constellations are visible as the year progresses. For example, in London, the constellation of Orion is always visible in the evenings of winter months of January and February. Scorpius is visible in the summer months of June and July around midnight.

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.

The descriptions below are for viewers in the Northern Hemisphere, especially London and the United Kingdom.
The planetary information is valid for any location at the same or similar Latitude (51.5°N).
Note that a degree in the sky is twice the apparent diameter of the Moon.

All times on this page are London (UK) times. This is normally GMT (Greenwich Mean Time also known as Universal Time).
In the United Kingdom, the clocks go forward by one hour for British Summer Time (BST) between mid March and late October.
A 24 hour clock is used so that 7pm is written 19:00.

September 2010

Mercury

Mercury passes through on 3rd.

The planet then moves rapidly into the morning sky becoming visible before dawn from about the middle of the month for about two and a half weeks. This is the best morning appearance of Mercury in the UK.

Mercury will be rising over an hour and a half before the Sun. Look for it low above the Eastern horizon just as the sky is beginning to get light. The planet gets brighter during the second half of the month, outshining all stars apart from the brightest.

Venus

Venus sets an hour after the Sun at the beginning of the month and becomes lost in the evening twilight after the first week.

This ends the planet's evening aqpparition for 2010. Venus will make a spectacular morning appearance in the final two months of the year.

Mars

Mars is too close to the Sun to be visible.

Jupiter

Jupiter is at on 21st and is then visible all night long.

During September the planet is at its closest to the Earth and is the brightest object in the sky, three times brighter than any star. It shines with an off-white steady light. Jupiter is close to the Full Moon on 23rd.

Through a good pair of binoculars or a telescope, the planet's four large moons are easily visible resembling little stars in a straight line with the planet. Their position changes from night to night.

Jupiter is among the faint stars of Pisces.

After opposition Jupiter will remain visible in the evening sky until next Spring. The next opposition will be in October 2011 (they occur every 13 months).

During September Jupiter will be very close to the planet Uranus. For a few days around the 21st the two planets will be visible in the same field of view in 8 by 50 binoculars. Uranus will be above Jupiter and on the limit of naked eye visibility from a clear, dark, moonless sky. Its green hue may be glimpsed. Its closeness to Jupiter is a good opportunity to locate this usually difficult-to-find and not-normally-visible planet which was unknown to the ancients and discovered in 1781.

Saturn

Saturn is too close to the Sun to be observed.

The Sun

The Sun continues moving southwards and the days continue getting shorter. The length of day is 13h 34m on 1 September. This decreases to 11h 41m by the end of the month. Sunset is at 7:47pm on the first of the month. The 7pm sunset is reached on 22 September - it is at 6:41pm by end of the month.

At 04:09 (London time) on 23 September, the Sun crosses the on its southward journey. This is the Autumn Equinox in the Northern Hemisphere.

The word Equinox comes from two Latin words meaning equal night. This is only one of two days in the year that the length of day and night are equal all over the world and the Sun rises and sets due East and West respectively. In fact, because the Sun takes a couple of minutes to rise or set and because of atmospheric refraction, the length of the day is always longer than expected. The twelve hour day actually occurs on the 25th.

On the Autumn Equinox, autumn begins in the Northern Hemisphere; the Sun is directly overhead at noon on the equator. On the North Pole, the Sun sets after six months of daylight; at the South Pole the Sun rises after six months of night.

The time of midday (when the Sun is due South and a sundial shadow is at its shortest) occurs at 1pm at the beginning of the month (this is 12 o'clock GMT - remember the clocks are still an hour forward) but moves rapidly back to 12:51 by the end of September.

This has the effect of shunting the diminishing amount of daylight towards the morning.

The Sun enters Virgo on 15th. Virgo is the largest of the Zodiac constellations and the Sun will take over 40 days to pass through it.

Note that the Zodiac dates do not tie in with astrology as astrologers are using dates from two thousand years ago.

Date	Sunrise	Midday	Sunset	Length of Day	Sun's Noon Altitude	Notes
01-Sep	06:13	13:00	19:47	13h 34m	46.7°	Morning Half Moon
02-Sep	06:14	13:00	19:45	13h 30m	46.4°
03-Sep	06:16	13:00	19:43	13h 26m	46.0°
04-Sep	06:18	13:00	19:40	13h 22m	45.6°
05-Sep	06:19	12:59	19:38	13h 19m	45.3°
06-Sep	06:21	12:59	19:36	13h 15m	44.9°
07-Sep	06:22	12:58	19:34	13h 11m	44.5°
08-Sep	06:24	12:58	19:31	13h 07m	44.1°	New Moon at 11:30
09-Sep	06:25	12:58	19:29	13h 03m	43.8°
10-Sep	06:27	12:57	19:27	12h 59m	43.4°
11-Sep	06:29	12:57	19:25	12h 55m	43.0°
12-Sep	06:30	12:57	19:22	12h 52m	42.6°
13-Sep	06:32	12:56	19:20	12h 48m	42.2°
14-Sep	06:33	12:56	19:18	12h 44m	41.9°
15-Sep	06:35	12:56	19:15	12h 40m	41.5°	Evening Half Moon - Sun enters Virgo
16-Sep	06:37	12:55	19:13	12h 36m	41.1°
17-Sep	06:38	12:55	19:11	12h 32m	40.7°
18-Sep	06:40	12:55	19:08	12h 28m	40.3°
19-Sep	06:41	12:54	19:06	12h 24m	39.9°
20-Sep	06:43	12:54	19:04	12h 20m	39.5°
21-Sep	06:45	12:54	19:02	12h 16m	39.2°	Jupiter at Opposition
22-Sep	06:46	12:53	18:59	12h 12m	38.8°
23-Sep	06:48	12:53	18:57	12h 09m	38.4°	Full Moon at 10:17 - Moon close to Jupiter - Equinox at 04:09
24-Sep	06:49	12:53	18:55	12h 05m	38.0°
25-Sep	06:51	12:52	18:52	12h 01m	37.6°
26-Sep	06:53	12:52	18:50	11h 57m	37.2°
27-Sep	06:54	12:52	18:48	11h 53m	36.8°
28-Sep	06:56	12:51	18:45	11h 49m	36.4°
29-Sep	06:58	12:51	18:43	11h 45m	36.0°
30-Sep	06:59	12:51	18:41	11h 41m	35.7°

The Moon

Date	Notes
1	Half Moon
8	Perigee (Moon at its nearest to Earth, 357,192km)
8	New Moon
15	Half Moon
21	Apogee (Moon at its furthest from Earth, 406,167km)
23	Full Moon (Harvest Moon)
23	Moon close to Jupiter

The nearest Full Moon to the Autumn Equinox is called the Harvest Moon. In 2010 the Full Moon occurs on 23 September just six hours after the Equinox.

What is the Harvest Moon?

The Moon travels around the Earth. Because of this movement it rises (on average) 52 minutes later each day. This figure of 52 minutes is only an average. It can be as little as 12 minutes - or as much as nearly 2 hours. In September, the 12 minutes delay occurs around the time when the Moon is full. This creates the situation where the Moon appears to rise at roughly the same time on successive nights around the time of the September Full Moon. This month is normally the time of harvest so the Moon's light allowed the harvesters to stay out after sunset, hence the name.

Look at these moonrise times (BST) for London around 23 September 2010

21st 16:58
22nd 17:12
23rd 17:26
24th 17:42
25th 18:01

Notice how it takes five days for the Moon's rising time to move forward by just over an hour.

Compare with moonrise times (GMT) around the time of the Full Moon in March (30th in 2010).

28th 19:59
29th 21:18
30th 22:27
1st May 23:24
3rd 00:07

Notice how the Moon is rising well over an hour later each day. In the five days, the time of moonrise changes by more than four hours.

Next Month

Jupiter in the evening sky. Saturn in the morning sky. Clocks go back to GMT.

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

Sources: Astronomy Now magazine, Cybersky, USA Naval Observatory and UK Nautical Almanac Office.

Books From Amazon.co.uk

Observers' Glossary

The Observers' Glossary

An explanation of the terms used by sky observers. Includes descriptions of how the objects of the solar system behave in the sky as seen from Earth (especially the Northern Hemisphere high latitudes).

KryssTal Related Pages

The Solar System

Tables and data about the Sun, Earth, Moon, planets, asteroids and comets. All terms used are explained.

Measuring The Stars

An account of how various properties of stars can be measured by studying starlight. Includes brightness, distance, luminosity, temperature, mass, radius, density and an introduction to the H-R Diagram.

The Brightest Stars

A table containing a list of the 20 brightest stars in the Earth's sky. Explanations of all the associated terms like magnitude, spectral type and radial velocity.

Eclipses, Transits and Occultations

A detailed account about eclipses, transits and occultations. These are irregular phenomena that can be observed in the sky. Includes eclipse trips around the world with photos and well as photos of recent transists of Mercury and Venus.

The Scale of the Universe

In astronomy distances and times are confusing because of the large numbers of zeros in the figures. This is an easy-to-understand scaling of the Universe in both space and time. Distances in space are represented by the time light takes to travel there. The chronology of the Universe is compared to a real year.

A Brief History of Astronomy

How humanity came from believing Creation Myths to postulating the Inflationary Big Bang Theory. The key stages in our understanding of our place in the cosmos and the people who broadened our understanding.

The Calendar

What do we mean by the words day, week, month, year? Who invented our calendar? When did the third millennium begin? The relation between time and astronomy.

Gravity

The force that moves apples and planets. A short introduction to the ideas of Kepler and Newton that culminated with the theory of Universal Gravitation.

Spherical Trigonometry

A look at the mathematics of a sphere with a section on sundials and the equation of time.

External Skywatching Links

These links will open in a separate window

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.