Night sky viewing with a planisphere

If you are not inclined, or don't have the money, to purchase either a pair of stargazing binoculars or a telescope, but still wish to learn about the night sky by actually going outside at night and gazing skyward, you could find no more practical and useful astronomy accessory than a planisphere.

A planisphere is a handheld, two-layer, circular device (a bottom layer of boxboard-type material, with a plastic top layer) that shows the night sky at any time on any given night of the year.

The top plastic section is joined to the bottom section at the centre via a grommet, which permits the two sections to rotate around one another. This section is typically imprinted with an opaque area in which there is a clear, circular window, set slightly off centre. Around the edge of the opaque section are 24 hourly markings denoting the day/night cycle.

The bottom section, often referred to as the star map layer, shows the stars and constellations of the night sky. Along its outer edge are the months of the year, usually broken down further into two-day sections.

Years are not marked on the planisphere simply because the night sky does not change (by any significant degree), with respect to the stars shown on the planisphere, from year to year, i.e., the night sky you look at now in January 2022, will be the same night sky you look at next January, and for countless years to come.

The clear window section will usually have the four directions (North, South, East and West) marked around its edge, although some may only have East and West marked, in which case, you can still identify North and South.

Some planispheres may also indicate the constellations with lines connecting various stars. Some also name the constellations, the brightest stars, star clusters, and certain deep sky objects. Hand-held planispheres do not show the position of the planets or the Moon.

Online, user-friendly planispheres, however, allow the viewer to add or remove numerous digital layers, showing a variety of celestial features in the night sky.

Using the planisphere is quite simple. First locate the month and date that corresponds to the night you are planning on looking at the sky, e.g. Jan. 24. Next, rotate the upper layer of the planisphere so that the time you are planning to look at the night sky (e.g., 10 p.m.) on the edge of the opaque section lines up with the date you have selected.

Once you have done that, hold both top and bottom layers firmly together (so they don't rotate further).

Before proceeding further and attempting to equate what shows on the planisphere with the stars above you, visually find the North Star (Polaris). First, locate the Big Dipper asterism in the constellation of Ursa Major — the Great Bear.

If you don't know what the Big Dipper looks like, it is an arrangement of stars that resembles a large dipper, ladle or pot, with four stars representing the dipper's bowl, and three-stars the handle. It is one of the most recognizable and distinctive star groupings in the night sky.

Once you have located the Big Dipper, draw a line through the two stars at the end of the dipper's bowl (known as the Pointer Stars), starting at the bottom star and extending a distance around four times the distance between the Pointer Stars. You will come upon a medium bright star sitting almost all by itself in the night sky; this is Polaris — the North Star.

Close by Polaris is the celestial North Pole, around which the night sky appears to pivot (stars will rise in the eastern sky, move across the sky, and set in the western sky).

Once you have found Polaris, draw an imaginary line down to the horizon; this is north on a topographical map. From there, you can find the three remaining topographical directions (south is behind you, east to your right, and west to your left).

Still holding the planisphere firmly, orient it such that the grommet (which marks Polaris and, for all intents and purposes, the North Pole on the planisphere) corresponds to the location of the North Star/Pole. You will find that this usually results in the North edge of the planisphere's clear window pointing downward, as Polaris is about a quarter of the way up the sky (in the Northern Hemisphere).

You have now correctly oriented the planisphere with the night sky above you.

The clear window on the planisphere represents a horizon-to-horizon (in all directions) picture of the night sky that you are viewing at that time. Think of it as taking the arching, half-sphere of the night sky above you, and compressing it into a flat, pictorial representation of the night sky that you can immediately look at.

By rotating the upper layer around so that the time is adjusted as the night progresses, you will be able to find different constellations and stars as they rise in the east and move across the night sky to the west.

If you wish to look at the stars in the southern part of the night sky, simply orient yourself and the planisphere accordingly; i.e., if looking south, the North part of the clear window would be pointing upward.

You can purchase planispheres on-line (simply Google planispheres) or at any bookstore that sells astronomy books. I would advise buying a good quality one with a plastic coating (they last longer than paper ones and resist the dew that invariably comes with nighttime viewing) and be sure to purchase one that approximates your geographical latitude.

Most planispheres have instructions on how to use it on the back.

Mercury and Saturn are both too close to the Sun right now to be observed. Venus (mag. -4.5, in Sagittarius — the Archer) and Mars (mag. +1.4, in Ophiuchus — the Serpent-bearer) can both be found low above the southeast horizon around 6:30 a.m. AST (7:00 a.m. NST) — dim Mars to the left of brighter Venus — before both fading from view as dawn breaks around 7:20 a.m. AST (7:50 a.m. NST). Look for Mars, just above the teapot asterism in Sagittarius — the Archer, four degrees to the upper left of the slender, crescent Moon on the morning of Jan. 29, just before sunrise. Jupiter (mag. -2.1, in Aquarius — the Waterbearer) is visible 20 degrees above the southwest horizon by about 5:25 p.m. AST (5:55 p.m. NST), before sinking towards the horizon and setting around 7:45 p.m. AST (8:15 p.m. NST).

Until next week, clear skies.

EVENTS

Jan. 25: Last Quarter Moon Jan. 29: Mars four degrees to upper left of crescent Moon (pre-sunrise)