{"id":31730,"date":"2022-05-09T08:54:20","date_gmt":"2022-05-09T08:54:20","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=108208"},"modified":"2022-05-09T09:22:12","modified_gmt":"2022-05-09T09:22:12","slug":"how-to-navigate-the-night-sky-like-an-astronomer","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/how-to-navigate-the-night-sky-like-an-astronomer\/","title":{"rendered":"How to navigate the night sky like an astronomer"},"content":{"rendered":"

<\/p>

By Jane Green\n \t\t<\/p>

Published: Monday, 09 May 2022 at 12:00 am<\/p>

\n\n

T<\/span>he early Greeks believed that stars were embedded in an enormous sphere rotating around Earth.<\/p>\n

Today we know that stars and all the other celestial objects lie at varying distances, and they only appear to move from east to west as a result of Earth itself spinning on its axis from west to east.<\/p>\n

However, in order to describe the positions and movements of stars above our horizon, astronomers still use this vast imagined shell. It is called the celestial sphere.<\/p>\n

Projected onto this sphere is a grid system of reference points and lines, which are similar to maps used on Earth.<\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
TThe celestial sphere: Earth sits at the centre of the system, with the stars moving as if \u2018fixed\u2019 to the inside of a vast imagined shell.<\/span><\/figcaption><\/div>\n

Imagine Earth as a balloon with reference points and lines mapped on its surface.<\/p>\n

Inflate the balloon until it fills the celestial sphere and Earth\u2019s equator becomes the celestial equator, the celestial poles align with Earth\u2019s rotational north and south poles, and the lines of longitude and latitude become celestial coordinates<\/a> \u2013 respectively right ascension (RA) and declination (dec.).<\/p>\n

These coordinates are found on star charts and planetarium software apps such as Stellarium<\/a> or Star Tracker used by stargazers around the world.<\/p>\n

In fact, there are numerous stargazing and astronomy apps<\/a> that can do this for you.<\/p>\n

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Planetarium apps like Stellarium are helpful for providing the celestial coordinates of targets like Polaris (Alpha (\u03b1) Ursae Minoris)<\/span><\/figcaption><\/div>\n

Understanding celestial coordinates<\/span><\/strong><\/h2>\n

You may have heard terms like \u2018declination\u2019 and \u2018right ascension\u2019 \u2013 or \u2018dec.\u2019 and RA\u2019 \u2013 being used by astronomers, but what do they mean and how are they used to navigate the night sky?<\/p>\n

Declination<\/strong><\/h3>\n

Declination in the night sky is mapped in the same way that latitude is measured on Earth: in degrees, arcminutes and arcseconds north and south of the celestial equator.<\/p>\n

Measurements increase from 0\u00b0 on the equator to 90\u00b0 at the poles. Declination north of the celestial equator is detailed as positive (+), while south is negative (\u2013).<\/p>\n

As examples, Polaris (Alpha (\u03b1<\/span>) Ursae Minoris), the pole star \u2013 or north star<\/a> \u2013 for the northern hemisphere, has a declination of just over +89\u00b0 and Rigel (Beta (\u03b2<\/span>) Orionis), the blue supergiant star marking the Hunter\u2019s foot in the constellation of Orion<\/a>, is positioned at \u20138\u00b0 15 arcminutes below the celestial equator.<\/p>\n

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Betelgeuse and Rigel in Orion. Credit: iStock<\/span><\/figcaption><\/div>\n

Right ascension<\/strong><\/h3>\n

The celestial equivalent of Earth\u2019s longitude is called right ascension (RA) and although it can be shown in degrees, it is normally expressed in units of time: hours, minutes and seconds.<\/p>\n

24 hours equals 360\u00b0 and one hour of RA equals 15\u00b0 of arc. The RA of the star Polaris is therefore described as 2 hours, 59 minutes and the RA of Rigel is 5 hours and 15 minutes.<\/p>\n

Every object in the heavens can be located by using these RA and dec. coordinates.<\/p>\n

The zero (0) line of RA \u2013 the celestial equivalent of Earth\u2019s Greenwich Meridian of longitude \u2013 passes through the point where the Sun crosses the celestial equator at the first moment of spring in the Northern Hemisphere, the vernal equinox.<\/p>\n

Hours in RA are measured eastward from this point until 23 hours, 59 minutes is reached. One minute later and we return to 0 hours.<\/p>\n

Dividing the celestial sphere is a line denoting the ecliptic<\/a>, marking the apparent <\/span>path of the Sun across the background stars \u2013 located by tracing a line between one or more planets and the Moon.<\/p>\n

Tracking stars on the celestial sphere<\/strong><\/h2>\n
\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Long exposure images show the apparent movement of the stars in the night sky. Credit: Iain Todd<\/span><\/figcaption><\/div>\n

Which stars can be seen and how they appear to move with respect to your location can be conceptualised using the celestial sphere.<\/p>\n

At the North Pole (90\u00b0), the North Celestial Pole corresponds with the zenith \u2013 the point directly overhead.<\/p>\n

The celestial equator is now parallel with the horizon and as stars move along a path parallel to the horizon, only those in the northern half of the celestial sphere are visible.<\/p>\n

At Earth\u2019s equator (0\u00b0) you can see all the stars since the celestial equator <\/span>arcs from horizon to horizon via the zenith and the North and South Celestial Poles lie on the horizon.<\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
A diagram showing the difference between altitude and azimuth<\/span><\/figcaption><\/div>\n

The stars rise straight up in the east and sink straight down in the west. Between these extremes, part\nof the sky always remains invisible.<\/p>\n

So if you\u2019re observing at either pole the stars will circle overhead and never set \u2013 they are circumpolar \u2013 but the stars circling the opposite celestial pole will remain invisible. <\/span><\/p>\n

Your hands and fingers held at arm\u2019s length are ideal measuring tools for navigating the celestial sphere; an outstretched hand measures around 22\u00b0 \u2013 the width of the Plough \u2013 and your little finger equals 1\u00b0 \u2013 a full Moon\u2019s width.<\/p>\n

Telescopes and star coordinates<\/strong> <\/span><\/h2>\n

Different telescope mounts<\/a> are used to guide telescopes using RA and dec. coordinates<\/span><\/p>\n

Altaz mounts<\/strong><\/h3>\n
\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
The various parts of an altaz mount.<\/span><\/figcaption><\/div>\n

Altaz mounts have two axes of rotation: one moves in elevation \u2013 up and down \u2013 0\u00b0 to 90\u00b0 (altitude); and the other moves in a 360\u00b0 circle parallel to the horizon (azimuth).<\/p>\n

Equatorial mounts<\/strong><\/h3>\n
\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
On an equatorial mount, adjust the declination (dec.) axis to move your telescope in a north\u2013south direction.<\/span><\/figcaption><\/div>\n

Equatorial mounts are like altaz mounts, but tilted at an angle based on the observer\u2019s longitude, which corresponds with Polaris\u2019s altitude.<\/p>\n

They compensate for Earth\u2019s rotation by having one rotational axis \u2013 the RA or \u2018polar\u2019 axis \u2013 parallel with Earth\u2019s axis of rotation.<\/p>\n

They are moved by manual adjustment using RA (right ascension) and dec. (declination) setting circles, or a small electric motor on the axes, within the RA and dec. coordinate system, which allows all-night tracking of objects on the celestial sphere.<\/span><\/p>\n

\"""\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
On an equatorial mount, adjust the right ascension (RA) axis to move the telescope in an east\u2013west direction.<\/span><\/figcaption><\/div>\n

Fully motorised and computerised Go-To mounts <\/b>remove the need to know RA and dec. coordinates, as brighter objects are listed on a database accessed at the press of a button, but RA and dec. are still useful for locating fainter targets.<\/p>\n

This guide originally appeared in the May 2022 issue of <\/strong><\/em>BBC Sky at Night Magazine<\/strong>.<\/strong><\/em><\/p><\/body><\/html>\n

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