Observing Planner

Which planets stand where during a night, how bright is the moon – and which galaxies, nebulae and star clusters are most worthwhile at the chosen location? Everything is computed in your browser; no data are sent to other servers. Whether the night will be clear is shown by Astro Weather.

How to read the observing planner

Location and night: choose a location and the date of the evening the night starts on; the arrows step one night forwards or back. All times are in the location’s local time, across a daylight-saving switch too. For custom coordinates the page does not know the time zone and uses your browser’s instead – the status line says so. As on Astro Weather, custom coordinates are protected by a password.

The line below the settings sums up the night: sunset, the astronomically dark period and the part of it with the moon down, how much of the moon is lit and when it rises (↑) and sets (↓), and sunrise.

Planets and moon

The panorama shows the sky as seen from the location: looking south (north on the southern hemisphere), with the horizon and the compass points along the bottom. The dashed lines are the tracks of the moon and planets after sunset; the time slider moves them to where they stand at that time, and the colour of the sky shows the twilight. Brighter planets are drawn larger, planets below 10° paler, as they are hard to see in the haze above the horizon. The moon appears in its actual phase, its lit side facing the sun. Below, one bar per body shows when it is above the horizon – the stronger, the higher, weakened in twilight –, with its best altitude in darkness and its brightness on the right. Clicking the bars sets the time.

The best objects of the night

The list picks, out of 168 well-known galaxies, nebulae and star clusters, the 15 that are most worthwhile at the location this night. Objects that do not climb at least 20° above the horizon during darkness are left out. The two buttons choose what the rating is for:

• For imaging what counts most is how long and how high an object stands during astronomical darkness: an hour at 60° or more counts fully, below 20° not at all – the higher, the less air in between. A bright moon nearby takes points off, more for galaxies and faint nebulae than for star clusters. Very small objects get slightly fewer points, as without a long focal length they only fill a few pixels.
• For the eye brightness and surface brightness count – a large, faint object is harder to see than a small, bright one –, together with the greatest altitude in darkness and the moon.

At locations near a city, such as the Public Observatory Hannover, faint galaxies and nebulae drop down the list, as the brightened sky swallows them first. The score column runs from 0 to 100; it compares the objects of the same night and is no guarantee of success. Clicking an object marks it in the panorama as a light-blue ring with its track and sets the time to its best moment; if it stands outside the view, the panorama turns towards it. A second click removes the mark. On narrow screens every object appears as a card of its own.

Terms in brief

• Astronomically dark: the sun is more than 18° below the horizon; the sky is as dark as it gets at that place.
• Brightness (mag, magnitude): the smaller the number, the brighter. Under a dark sky the naked eye sees objects down to about +6, binoculars down to about +9.
• Surface brightness: how concentrated the light is. A galaxy of magnitude +8 whose light is spread over half a degree is much harder to see than a star cluster of the same brightness.
• Size: the largest diameter on the sky in arcminutes (′) or arcseconds (″). The full moon measures about 30′.
• Galaxy: a separate island of stars far beyond the Milky Way, mostly faint and extended.
• Emission nebula: glowing gas excited by young, hot stars – mostly red in images.
• Reflection nebula: dust reflecting the light of nearby stars – mostly bluish.
• Dark nebula: dust hiding the light behind it and showing as a dark shape against glowing gas.
• Planetary nebula: the shed shell of a dying star, often small and bright.
• Supernova remnant: the debris cloud of a stellar explosion.
• Globular cluster: a dense ball of hundreds of thousands of old stars.
• Open cluster: a loose group of young stars, often lovely in binoculars already.

Where the values come from

Sun, moon and planets are calculated by your browser. The moon follows the main orbital perturbations after Meeus and agrees with the U.S. Naval Observatory on rise and set to about a minute. The planets come from JPL’s (NASA) orbital elements, their brightness from the model by Mallama and Hilton (2018); both were checked against the Horizons ephemeris service: positions agree to better than a tenth of a degree, brightness to about a tenth of a magnitude.

The object catalogue holds every Messier object except M40 (a double star) and M73 (a small group of stars), plus well-known NGC and IC objects, including southern ones such as Omega Centauri and Centaurus A. The coordinates come from the SIMBAD astronomical database in Strasbourg; brightness and size are rounded guide values.

The scores of the list are a weighting of my own from these data, and so is classing a location as near a city. The weather does not feed in – whether the night will be clear is shown by Astro Weather.

Coordinates: SIMBAD database, operated at CDS, Strasbourg, France · Planetary orbits: JPL (NASA) · Planetary magnitudes: Mallama & Hilton (2018). Scores and location classes are derived from these.