Planning your observation night using Right Ascension data, or, how to be a top-class geek

A random walk through the sky

When I first started in astronomy (back at school), our astro club's observation nights were unplanned affairs. We'd turn up, maybe have a quick squiz at Norton's Star Atlas to see what's up, and then grab a scope. We'd wander randomly from one target to another, exploring the sky on our own.

With a plan, you don't miss a target

Inevitably though, the day afterwards, someone would start talking about the Lagoon Nebula or some other cool target. When we'd find out where it was so we could see it, we'd find it was just setting in the evening and we'd have to wait six months to see it.

(Geek bullying - find some really cool object in the sky, then wait to tell your victim about it until it's too close to the Sun to observe. How very Niles Crane.)

Observation nights always go better when you can plan your targets. And it's not just visual observers, either. Astrophotographers want to expose the target for as long as possible, and this time changes over the year, interrupted by sunrise and sunset.

So how do you make a plan?

What we needed was to know was how far what we were looking at had gone in their nightly march from east to west. Preferably, they'd still be in the east at the start of the night so they were conveniently located for us to see. Ideally, of course, they'd be at their highest point when we were watching them.

The idea is to get a list of targets that do all this.

I normally use Stellarium or for planning (of course, there are others). Once you have set your location, you can select a target and it'll tell you when it rises and sets. Compare that to when the sun rises and sets and you can figure out your window of observing opportunity.

In addition, can tell you what time of year is best for any object, given a bit of extra information such as when your object is highest in the sky (and you're looking through the least amount of atmosphere).

A bit of a shortcut - where's the antisun?

However, there's a simpler (and fairly rough) method that gives you about as much information as you'll need.

Sky charts use Right Ascension (RA) and Declination (Dec) to report locations of stars. RA shows how far east or west the star is (from an arbitrary point), and Dec shows how far north or south it is. Co-ordinates for stars don't change much, but the sun and planets move about, so their co-ordinates change over the course of the year.

If you know the RA of the Sun (look up a website like, or check out the Astronomical Society of Victoria's Astronomical Yearbook, 2021, p.44), you can calculate the "antisun" (the location furthest from the sun) by adding or subtracting 12 hours from the Sun's RA. (The Declination of the antisun is simply the negative of the Sun's Declination, but that's not relevant to this article.)

I've added a diagram showing the Sun's RA, and therefore the antisun's approximate RA for the first of each month in 2021 here.

The RA of the antisun will pass the local meridian (directly above you and north to south) at "local midnight", because that's when the sun is directly below you. I've added these times to my diagram in red. Targets around the antisun's RA will be furthest in time from both sunrise and sunset, and therefore have the longest window of observing opportunity. This gives the astrophotographers the longest time to photograph the target.

For visual observers, add or subtract three or four hours from the antisun RA and you'll get an observable range. You'll be able to get a reasonable amount of time on targets that fall in this range. If you want to start early and finish early, you can offset this. If you want to get more sophisticated, you can figure out the middle time point in your intended observation session and calculate the time when the Sun is directly below you at that time.

For a successful observation night, get targets with a close range of Right Ascension co-ordinates

This might be a bit hard to follow, but if you're planning an observation night, get a list of targets that have RAs within a few hours of the antisun's RA. They'll all be viable (as long as their Declinations don't put them below the horizon, of course).

My other piece of advice is begin your observations with the westmost target (with the lowest RA) or it'll set before you can get to it.

If you're an astrophotographer, find the RA of your target and compare it to antisun RA numbers in the chart. That'll give you a good idea of the best time of year to photograph the target - otherwise your exposure will be cut short by sunset or sunrise.

Geeks rule - eventually!


Post script

This is just a rough guide. There are loads of reasons why it's approximate, or things you might need to factor in. Some of these include:

  • Your location might not be the middle of your time zone 
  • Months have different lengths 
  • Every four years we need a leap day 
  • Observers like to go to bed at different times 
  • Declination of targets might mean it's too far North and therefore too low to see well