About These Calculations

Basis of Calculations
These pages calculate approximate observability of the planets for an observer in the southern UK. Calculations are based on Paul Schulyter's excellent web tutorial of how to calculate the positions of the planets (available here). Corrections for atmospheric refraction are after Saemundsson et al, for example as shown here. The planetary osculations (orbital parameters) are taken from Schulyter's page.

The asteroid orbital parameters are taken from the MPC daily datafile which is downloaded each night to my server. Asteroid orbital parameters are updated roughly every 100 days with different update dates for each asteroid group but their orbits are quite variable, especially the smaller ones, so do not expect much accuracy beyond 100 days.

Be sure to set your location first via the Switch Location page
Click here to go to the location page. Also do check the location before using the data.

What the Graphs Show

  • Best Visual Altitude - the highest altitude (culmination) of the planet on that particular day, if itis observable at all (zero means unobservable). If the planet culminates in daylight, then the higher of dawn or dusk altitude is used. Hover your mouse over the graph to see the actual best time. See note below for what 'observable' means exactly.
    NB: When observing at dawn, please take care that sunrise does not happen in the field of view of your scope or camera, or you will blind yourself.
  • Size and Magnitude - The approximate size and magnitude on the date shown. The brightness of inner planets takes into account their phase as well as distances. The brightness of Saturn's rings and their tilt is included. For asteroids and pluto, the data will be sketchy.
  • Download raw data - the raw data that went into the graphs is available to download. Although I calculate the data to many decimals, its really only accurate to two or three so I round it in the files.

    Observability, and Visual vs Imaging
    To be 'observable', I have assumed the sun must be below the horizon and the planet at least 5° above. A value of zero means the planet is unobservable, either too close to the Sun or not above the horizon during darkness. A non-zero value does not mean it will be easy to see ! Generally a planet will be unobservable if less than 10° from the Sun, and dangerous if less than 20° away. Thats why I made sure the Sun was below the horizon.

    The graphs show when the planet will be visible, for astro-imaging you really want the sun more than 15° below the horizon and the planet more than 20-25° above. I have also modelled this and will add it as a feature when I have time but for now you can estimate it by ignoring all data at dawn, dusk or where altitude is less than 20-25°. You can certainly image at lower altitudes but the results will be less wonderful.

    Adjusting for Other UK Locations
  • Either use the location switcher page, or just add (subtract) approx 1 degree from the altitude for every degree south (north) you live.
  • Adjust the time later for more western locations and earlier for more eastern locations. Its not easy to say how much I'm afraid, but around 15 mins per degree should be about right.

    How I Calculated the Data
    I modelled the orbits in Excel VBA for a fixed location (mine!), and tested it by comparing to Stellarium. I then rewrote the calcs in C++, added the ability to model different locations and created some code which pushed the numbers into javascript suitable for the graphing tool.

    Accuracy
    Do not expect sub-degree accuracy! There are many detailed perturbations which affect planetary positions, notably resonances between planets, the Sun and the Moon, which cause small deviations. I've modelled the biggest of these for Jupiter Saturn, Uranus and Neptune (and the Moon) as well as adjusting from geo to topcentric coordinates, but I'm not modelling things like wobbles of the Earth's axis, Earth's movement around the Earth-Moon barycentre, resonances between the inner planets and so on. I've spot-checked the numbers for all the planets at random dates out to 2026 and am generally within a degree or so altitude and 5-10 minutes time, but there are some larger errors especially for the asteroids.

    Coming Soon
    I hope to add the following shortly:
  • Model the brightness of Saturn's rings.Done.
  • add Pluto and a few minor planets or asteroids.Done.
  • move the calculations into C++ and make them dynamic.Done.
  • create dynamic webpages to allow people to calculate data for their own location.Done.
  • model the effect of the Moon on observability.