Transit times and declination charts

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Corrected charts for 2003 now uploaded! If you downloaded the charts before 31 Dec 02, then you got the 2002 charts by mistake.



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This page describes charts for the year 2003 that can be downloaded in Adobe Acrobat format or as a large GIF. These charts show on one side of A4, for each day in the year;

These charts enable you to decide on your chances of seeing a given planet on a given day in the current year - they help you answer the question "Is the planet far enough away from the Sun and high enough above my horizon to be able to see, and if so, can I get a good observation?". These charts require no batteries!

In the Northern hemisphere, the sphere of stars seems to revolve around the Pole Star, in the Northern part of the sky. Stars near Polaris never rise or set, but as you look further to the south, you will find stars which rise in the east and set in the west. Imagine a line on the sky drawn from the South point on your horizon, through the point directly overhead, to the North celestial pole and then down to your Northern horizon. This semi-circular arc is your meridian. The transit time for a star or other celestial object is the time the object crosses your meridian south of your zenith. This transit time also corresponds to the time when the object is highest in the sky for Northern observers, and is also known as culmination, or upper culmination in the case of a star which does not rise or set. Nick Strobel's excellent Astronomy without a Telescope page provides a good overview of the Celestial Sphere and the associated geometry.

Some stars do not stay in the same position on the Celestial Sphere. These wanderers are the planets, and their transit times will change from day to day. A chart of the transit times for the Sun, Mercury, Venus, Mars, Jupiter and Saturn will give you a good idea of the observability of these planets, and which constellation they are in on a given day of the year.

Download or print the charts

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The charts were produced in Microsoft Excel, and are available here in two forms: as an Adobe Acrobat file (for which you need a copy of the Adobe Acrobat reader), and as large GIF file. The Adobe versions provide much better print quality, and the whole point of these graphs is to print them onto paper.

Adobe Acrobat PDF files

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Combined chart (2 pages) in PDF format (about 40Kb each) | 2003 | 2004 | 2005 |

To print the Adobe format version, some browsers use the Adobe Acrobat reader as a plug-in, so following the link will load the reader. In this case, just hit print. With older browsers, you will need to save the Adobe Acrobat file to your computer, and then load it into the reader to print the file.

GIF files - 977 by 600 pixels 16 colour

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Transit times | 2003 | 2004 | 2005 |
Declinations | 2003 | 2004 | 2005 |

To print the GIF file, just

Overall patterns

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A look at the charts will convince you of the following general principles...

Looking at the 2003 charts we see

Using the charts

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These charts give the time of transit for your local meridian in local mean time. Most people now use a civil time referred to one of the recognised international time zones. If you want to squeeze the most accuracy from these charts, you need to know the difference between your local time and the zone time. If you are (say) 2.5 degrees West of your time zone meridian, your local time runs 10 minutes behind the zone time, and you must add the 10 minutes to the transit time in local time as found from your chart to get the transit time at your meridian in zone time. If East of your zone time meridian, then you must subtract the minutes.

To use these charts to the fullest, you will need to know

The transit time and declination charts both have the horizontal axis scaled in days of the year. Grid lines are placed every 30 days, and there are 'tick marks' for each 10 days.

Shows day number axis and how you can use the 30 day grid to approximate months

In practice, I tend to work to the nearest 10 day period on the days axis, and I tend to regard each major division as a month. This leads to a worst case error of 5 days at the end of the year - not too dramatic for most of the planets (but possibly important for fast moving Mercury). Under these circumstances, I don't bother with the correction for the difference between zone and local time.

Day numbers

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You can be a bit more precise by using the tables below to find the number of days in the year corresponding to the current date:

Leap year

                 Jan     0        Jul   182
                 Feb    31        Aug   213
                 Mar    60        Sep   244
                 Apr    91        Oct   274
                 May   121        Nov   305
                 Jun   152        Dec   335

To find the day number of any date in 2004, just add the date to the
number of days before the month given above. eg, May 26th 2004 is day
number 121 + 26 = 147

Non leap year

                 Jan     0        Jul   181
                 Feb    31        Aug   212
                 Mar    59        Sep   243
                 Apr    90        Oct   273
                 May   120        Nov   304
                 Jun   151        Dec   334

To find the day number of any date in 2003, just add the date to the
number of days before the month given above. eg, June 4th 2003 is day
number 151 + 4 = 155

Find the transit time

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To find the transit time of a given planet or the Sun, you just

  1. use the table above to find the day in 2000, or approximate the Month and day using the 30 day grid lines
  2. put a ruler parallel to the time axis at the day number on the date axis (horzontal axis)
  3. identify the point at which the ruler crosses the transit time line for the planet
  4. turn the ruler parallel to the horizontal axis
  5. read off the corresponding time on the time axis (vertical axis)
  6. add 4 minutes for every degree west of the meridian for your time zone, and subtract 4 minutes for every degree east
  7. and add an hour for daylight saving if needed.

The corrections in step 5 and 6 depend only on your location - you can work out the correction once and for all and write the figure on the charts.

Transit time for Mars on June 4th 2001

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Shows how to read off a transit time

Finding the declination and altitude at transit

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Reading off the declination from the declination chart is very similar to finding the transit time, except there is no need to correct for local time.

Once you have the declination, you can easily work out the altitude of the object at transit - if you know your geographical latitude. Altitude is the angle of the object above your local horizon, and this changes through the night.

Imagine you sliced the celestial sphere through your meridian like cutting a cauliflower in half. The resulting cross-section might look a bit like the one below - you are standing on a tiny Earth at the centre of the circle, and the plane of the diagram is your meridian.

showing the meridian and how to find the altitude at transit

The red lines represet the celestial equator and poles, the blue lines represent your local horizon and zenith. Starting at the North point on your horizon and working around the meridian anti-clockwise:

All of these angles must add to 180 degrees. A little work with angles will convince you that you can find the altitude of the planet at transit as follows;

  1. Take 90 degrees less your latitude
  2. Add the declination if positive
  3. Subtract the declination if negative
  4. The result is the altitude of the planet above the Southern horizon at the time of transit
  5. This altitude is the highest the object will reach on the night

Observations of the planets near the horizon mean that you are looking through a much thicker atmosphere than observations near the Zenith, and the seeing will be much worse than at higher altitudes. At an altitude of 30 degrees above the horizon, the air mass is 2, and at 15 degrees above the horizon the air mass is nearly 4 times as great as at the Zenith.

Declination and altitude at transit for Mars

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For Mars on June 4th 2001, the declination chart gives -26 degrees declination

showing how to read the declination from the chart

So Mars is well below the Celestial Equator, and will be low in the sky to the South. At my latitude (52.5 degrees North), we get

The demonstration version of SkyMap Pro, by Chris Marriott gives an altitude of 11 degrees 31 minutes for 0h 58m on June 4th 2001 from my Birmingham UK location.

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Last Modified: 31 Dec 2002
Keith Burnett