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Finding Stuff! by Martin Male

I learned to find my way round the night sky by finding a different constellation each night. Start with the Plough and work your way from there, perhaps finding Ursa Minor (that has Polaris in its tail) on the next night find Ursa Minor again, then locate Cassiopeia. On a fourth night find the first three again then look for say Cepheus: this way you will soon learn your way around.

 

Planisphere and Star Atlas

The Planisphere is a very useful tool to show you what part of the sky will be visible at any time. I suggest that you also get a star atlas, like the Cambridge Star by Wil Tirion, as this will help with finding objects as it has contains much more detail.

You will need to know how much of the sky you can see – the field of view (FoV) – with your telescope and eyepiece; the way to work this out is found elsewhere in this handout. Also your telescope will likely have a “Finderscope”; check your manual or the Internet to find out its FoV. Remember to check that it is aligned with the main tube (this is best done in daylight).

Once you know this, cut out a circle(s) of the relevant size from a sheet of paper to use as a “mask”; the right diameter will be found by using the scale found on the star chart.

Once you have your mask look at the atlas to see if there are any stars near your target and see how many “FoV’s” you need to offset to find it. It’s just a matter of practice! This is called “Star Hopping” and is a great way to find your way round the sky.

Planetarium programmes or Smartphone Apps often have some convenient means of displaying FoV, but even using their “night vision” view may lead to a loss of dark adaptation.

Many telescopes have a “GoTo” facility that can greatly help though it is very important to make sure the telescope is set up and aligned properly as what you are looking for might not always appear in the eyepiece after slewing.

 

Meridian

A meridian is an imaginary line drawn across the sky from north to south; this is local to your position and should not be confused with the Prime meridian at Greenwich.

When an object crosses this imaginary line (culminates) it will be at its maximum altitude. This is usually the best time to view it, as there will be the least amount of atmosphere between it and the viewer.

 

Right Ascension and Declination

RA (right ascension) and DEC (declination) are to the sky what longitude and latitude are to the surface of the Earth. Imagine a grid system projected onto the sky, with the point where the sky appears to rotate round being quite close to Polaris. The celestial equator is the part of the sky directly overhead on the equator of the Earth.

RA corresponds to east/west direction (like longitude), while Dec measures north/south directions, like latitude. RA is measured in hours, minutes, and seconds. 0 hours RA is by convention the right ascension of the sun on the vernal equinox, around March 21st.  The hours “ascend” in an easterly direction; what this means is that if an object at 3h 5m 38s is overhead now, in an hour’s time an object at 4h 5m 38s will be overhead, and so forth.

Declination is measured in degrees, arcminutes, and arcseconds; 60 arcminutes in a degree and 60 arcseconds in an arcminute.

Declination tells you how high an object will rise. So an object at +51° declination would pass directly overhead at Greenwich (51° latitude)

An object on the celestial equator (0°) would attain a maximum altitude of 39° from the horizon: 51 + 90 + 39 = 180.

Any object in the sky will have a unique position: Vega is at 18h 36m 56s, +38° 47m 0.1”; Antares, south of the celestial equator, never rises very high in the UK is at 16h 29m 24sec, -26° 25’ 55” (note how they are written to differentiate them!)

If you do the math’s (51 + 90 + 26 = 167, 180 – 167 = 13) you will see that Antares attains a maximum altitude of about 13° from Greenwich.