Finding Objects in the Night Sky Without Polaris

Finding Objects in the Night Sky Without Polaris

Find east, west and south from about 6° N with Orion's belt and the Southern Cross, then use coordinates to find anything else.

All times are Guyana time (America/Guyana, UTC-4, no daylight saving). Heights are for Georgetown unless Lethem is named.

Why Polaris is not much help from Guyana

Just like we use latitude and longitude to map locations on Earth, astronomers use a coordinate system to map positions in the sky. Understanding celestial coordinates is fundamental to finding and identifying objects in the night sky.

Most star guides start with Polaris, the North Star, because it sits close to the north celestial pole. Its height above the horizon equals your latitude. From Georgetown (6.8° N) Polaris is only about 6 to 7° up, and from Lethem (3.4° N) about 3 to 4°. Haze or trees often hide it, so this lesson, which builds on Introduction to Astronomy, uses other signposts.

Slide your latitude and watch Polaris: its height above due north is your latitude, give or take the small circle it swings round the pole.

Imagine the night sky as a giant dome overhead. This is called the celestial sphere: an imaginary sphere of infinite radius where all celestial objects appear to be located, with Earth at the centre.

Star trails over La Silla, Chile, showing the sky turning around the celestial pole. From Guyana the northern trails circle Polaris, just above the horizon. Credit: G. Lambert/ESO (CC BY 4.0).

Find your directions without Polaris

At arm's length a fist covers about 10° of sky and a thumb about 2°. Use them to judge every distance below.

Orion's belt: due east and due west

The three belt stars sit almost on the celestial equator. Mintaka, the western one, is at Dec -0.3°, so it rises within half a degree of due east and sets within half a degree of due west. The whole belt, about 3° long, rises within 2° of due east. It climbs almost straight up, passes about 8° south of the zenith, and drops almost straight down in the west. Look for it in the evening from about November to April: at 21:00 it is about 17° up in the east in mid-November and about 13° up in the west in mid-April.

Orion's Belt rising over the eastern horizon from Georgetown or Lethem: where Mintaka comes up, and how straight it climbs.

The Southern Cross and the Pointers: due south

The Southern Cross (Crux) is small: its long axis, from Gacrux at the top to Acrux at the foot, is about 6°. Alpha and Beta Centauri, the Pointers, sit beside it. From Georgetown Acrux climbs about 20° at most, and from Lethem about 23°.

The south celestial pole never rises from Guyana, and neither does Sigma Octantis, the faint star closest to it.

The Southern Cross and the Pointers over the southern horizon: when the cross stands upright, and the line from its long axis down to the hidden pole.

Why Polaris barely clears the northern horizon from Guyana, and how the Southern Cross finds south instead: rising, upright at 22:45 and leaning west on 15 April 2027 over Georgetown.

The Big Dipper, if you still want Polaris

From Big Dipper to Polaris is the classic northern hop. From Guyana Polaris is only about 3-7° up, so use a clear, flat northern horizon. Dubhe peaks about 35° up due north, at about 21:20 in mid-April; follow Merak to Dubhe and keep going about 29° (three fists) down to Polaris.

The celestial equator, the meridian and culmination

The celestial equator runs from due east, up to a point 6.8° south of the zenith (83.2° up, due south), and down to due west. From Lethem it passes 3.4° south of the zenith. A star on the equator is up for about 12 hours.

The meridian is the line from due north, through the zenith, to due south. A star is highest, or culminates, when it crosses the meridian. Its height then is 90° minus the gap between your latitude and its declination. If its declination is south of your latitude it culminates due south; if north, due north.

| Object | Dec | Highest from Georgetown | Highest from Lethem | |---|---|---|---| | Betelgeuse | +7.4° | 89° due north (nearly overhead) | 86° due north | | Orion's belt (Alnilam) | -1.2° | 82° due south | 85° due south | | Orion Nebula (M42) | -5.4° | 78° due south | 81° due south | | Sirius | -16.7° | 66.5° due south | 70° due south | | Galactic Centre | -29.0° | 54° due south | 58° due south | | Dubhe | +61.8° | 35° due north | 32° due north | | Acrux | -63.1° | 20° due south | 23° due south | | Polaris | +89.3° (J2000) | about 6-7° due north | about 3-4° due north |

The table, worked out again from each object's catalogue place, with the time each one crosses the meridian next tonight.

The sky turns 15° per hour. Earth spins once relative to the stars in 23 h 56 min 4 s, so each star rises about 4 minutes earlier each night, or about 2 hours earlier each month. Why Seasons Happen explains why.

Two ways to give a position

Altitude and azimuth: fixed to you

Alt-az is intuitive, but it changes every minute as Earth turns and differs from place to place.

Right ascension and declination: fixed to the stars

Right ascension (RA) is the celestial equivalent of longitude. It is measured eastward from the vernal (March) equinox point, the "0 hour" where the Sun crosses the celestial equator moving north. Lines of RA run from the north to the south celestial pole. RA is expressed in hours, minutes and seconds, from 0h to 24h, because the sky turns 360° in one sidereal day. So 1h = 15°, 1m = 15' and 1s = 15". Examples (J2000): Sirius is at RA 6h 45m 09s and Vega at RA 18h 36m 56s.

Declination (Dec) is the celestial equivalent of latitude. It is measured north (+) or south (-) of the celestial equator, in degrees, arcminutes and arcseconds, from -90° to +90°, and 0° is on the equator. Lines of Dec run parallel to the celestial equator. Example: Sirius has Dec = -16° 43'.

Worked example: finding the Orion Nebula

This tells us it is in the constellation Orion, slightly south of the celestial equator. With coordinates you can find it on a star chart, send a computerized telescope straight to it, or look it up in a smartphone app. By eye, go about 4° south of Orion's belt to the middle "star" of Orion's sword.

The Orion Nebula (M42) at coordinates RA 5h 35m, Dec -5° 23'. Credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team / Wikimedia Commons (public domain).

How the systems fit together

Equatorial coordinates (RA/Dec) are fixed to the stars, horizontal ones (alt/az) to Earth, and ecliptic ones to the solar system. Alt/az changes as Earth rotates, while RA/Dec stays (almost) constant. Converting between them needs your location and the time, plus spherical trigonometry that apps and calculators do for you. The "almost" is precession: Earth's axis wobbles over about 26,000 years, so coordinates slowly change. Catalogues therefore quote an epoch, usually J2000.0.

Tonight's sky with the altitude-azimuth grid, the right ascension and declination grid, or both: watch which one turns with the stars.

Hour angle: when will it be highest?

Hour angle (HA) = LST - RA, where LST (local sidereal time) is the RA on your meridian right now. When HA = 0h, the object crosses the meridian. A negative HA means it is still climbing in the east; a positive HA means it is past its highest point. Hour angle is also the link used in alt/az conversions.

Example: at 21:00 on 1 January 2027, the LST at Georgetown is about 3h 53m. For the Orion Nebula, HA = 3h 53m - 5h 35m = about -1h 42m. Precession has moved its RA to about 5h 37m since 2000, which makes it about -1h 44m. It is about 61° up in the east-south-east and culminates at 22:43, 78° up due south.

The lesson's worked example as a calculator: local sidereal time, hour angle, height and the time of culmination for the Orion Nebula and other targets.

What you can see from Guyana

The limit is 90° minus your latitude: 83.2° from Georgetown (6.8° N), 86.6° from Lethem.

So you see stars from both hemispheres, many pass nearly overhead, and southern sights such as Omega Centauri, which never rises from 45° N, are on show.

Quick Reference

| Object Type | Typical RA Range | Typical Dec Range | |-------------|------------------|-------------------| | High in the evening sky, December to March | 4h - 8h | -30° to +30° | | High in the evening sky, June to September | 16h - 20h | -30° to +60° | | Southern Objects | Any RA | -90° to 0° | | Circumpolar (from Georgetown) | Any RA | +83.2° to +90° |

The sky and the weather through the year

Northern (coastal) Guyana is wet mid-April to the end of July and mid-November to the end of January, and dry August to October and February to March. Southern Guyana has one wet season, April to August.

For Milky Way timing, see Milky Way Season Near the Equator.

Star-hopping

Star-hopping means moving from a bright star you are sure of to an unseen target, one pattern at a time, measuring each jump with your hand.

The lesson's hops drawn on the real stars, with each jump measured.

Deep Sky Objects Introduction has more targets.

Charts, apps and telescopes

Tools on this site

Common beginner mistakes

Kit that helps

Practice

  1. On a clear evening from November to April, note where Orion's belt rises or sets against your horizon. That is due east or due west.
  2. Between April and July, wait for the Southern Cross to stand upright and note the landmark below it. That is due south.
  3. Pick Jupiter or Saturn in Planets Visible Tonight, note its peak time, altitude and direction, then find it with a star chart and your hand.

A local astronomy club or star party is a good place to learn hops from experienced observers.

Check yourself

From Georgetown, which way are you facing when Orion's belt is rising?

Due east, within about 2°. The belt sits almost on the celestial equator.

The Southern Cross is standing upright. Which way are you facing?

Due south. The cross is on the meridian.

How high does Sirius (Dec -16.7°) get from Georgetown?

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