Dark Window

Go / no go for deep sky

Polaris position finder

See where Polaris (or Sigma Octantis in the southern hemisphere) sits around the celestial pole right now, as a position on a clock face for your location and time. Use it to set up an equatorial mount with a polar scope, or to get close by eye before you refine.

Hemisphere
Time

Times use your device’s clock and time zone, so the time you pick is your own local time. Nothing is sent anywhere.

// Pole star position

Enter your latitude and longitude, or tap “Use my location”, to see where Polaris sits.

// Phone compass (optional)

Another way to find true north: your phone’s compass, corrected with the declination above. It is good to a few degrees, so sighting on the pole star is usually easier.

No declination entered yet, so the needle shows magnetic north, which can be several degrees off. Enter it in the box above.

How to use it

  1. 1

    Set the latitude

    The celestial pole sits as many degrees above your horizon as your latitude. Set your mount's altitude scale to the value shown above, and make sure the mount is level.

  2. 2

    Point roughly at true north

    Aim the polar axis at true north (true south in the southern hemisphere), not magnetic north. A compass points to magnetic north, which can be many degrees away depending on where you live, so correct it for your local magnetic declination (a quick web search gives it for your town) or sight on Polaris.

  3. 3

    Read the clock position

    Polaris is not exactly at the pole. It circles it once a day, so the position changes with the date and time. The clock face above shows where it sits right now around the center.

  4. 4

    Match it in the polar scope

    Turn the right ascension axis (or the reticle, on mounts that allow it) so the reticle's 12 o'clock mark is straight up. Then use the altitude and azimuth adjusters to put Polaris on the small circle at the clock position shown. The view above starts on Polar scope (SA 2i / GTi), which matches the upside-down image in those mounts' polar scopes. Switch to Naked eye only if you are sighting without a scope.

  5. 5

    Fine-tune if you need to

    Reticle alignment gets you close, usually within a few arcminutes. For long exposures at long focal lengths, finish with a drift alignment or a plate-solving polar alignment routine in your imaging software.

  6. 6

    Turn on sidereal tracking

    Once you are aligned, start the mount tracking at the sidereal rate. Sidereal means "measured by the stars": a sidereal day is the 23 hours 56 minutes the Earth takes to turn once relative to the stars, a few minutes shorter than the 24-hour solar day. At the sidereal rate the mount turns its polar axis against the Earth's rotation, about 15 arcseconds every second, so stars stay put in your frame. On the Star Adventurer 2i and GTi, choose the sidereal rate in the SynScan Pro app or set the mount's switch to it. Tracking at the solar or lunar rate instead makes stars trail.

Questions

Why isn't Polaris exactly at the pole?
It is about two-thirds of a degree away, roughly one and a quarter Moon widths. It circles the true pole once every sidereal day, like the hand of a clock, which is why its position on the reticle depends on the time and your longitude. The gap is slowly shrinking and will be smallest around the year 2100.
What if I'm in the southern hemisphere?
There is no bright pole star in the south. This tool uses Sigma Octantis, a faint star (magnitude 5.4) about a degree from the south celestial pole. It needs a dark sky and usually a polar scope or binoculars. Many southern imagers use the surrounding stars of Octans or a plate-solving routine instead.
Do I need the polar scope view or the naked-eye view?
If you have a Sky-Watcher Star Adventurer 2i or GTi, use Polar scope (SA 2i / GTi), which is the default. A polar scope shows the sky upside down, so the reading is turned by half a turn compared with what your eyes see. It matches the Polar Clock in the SynScan Pro app, so if you already use that, the two should agree closely. Use Naked eye only when you are sighting without a polar scope. If your reading is about six hours different from another polar clock you trust, switch the view.
What does sidereal mean?
It means measured against the stars rather than the Sun. The Earth turns once relative to the stars in about 23 hours 56 minutes (a sidereal day), and the Sun takes about 24 hours to return to the same spot because we also move along our orbit. Sidereal tracking drives your mount at the stars' rate, 360 degrees per sidereal day or about 15 arcseconds per second, which cancels the Earth's rotation and keeps stars still in your images. Turn it on after you polar align, and leave it on while you image.
How accurate is this?
The position is calculated from the star's coordinates, moved forward to today's date to account for the slow wobble of the pole. It ignores effects smaller than an arcminute or so, which moves the clock angle by about a degree at most. That is finer than a reticle can be set by eye. The time comes from your device's clock, so keep it set correctly.

Open the Dark Window planner to check tonight’s GO / NO-GO verdict before you set up.