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Basic observing skills.

It is often thought that a telescope is essential for observing and certainly, anything that enhances human vision is extremely useful to the astronomer. A simple pair of binoculars is an excellent too for exploring the night sky. Besides being far cheaper than a telescope, they are less cumbersome and easier to use, especially if sturdily mounted on a tripod. Regardless of the kind of optical aid one uses, the following basic knowledge and skills are required.

Angular separation

Distances in the night sky are measured in degrees (or finer divisions). From the horizon to the point overhead (zenith) is 90°. A fist at arm's length covers about 10° of sky; a fingertip about 1° (See back cover). A degree is divided into 60 arcminutes (60'). The Sun and Moon are each 1/2°, or 30' wide. One arcminute, which is about the resolution of the human eye, is divided into 60 arcseconds (60"). For example, Jupiter's disk is about 40" across, which is smaller than the human eye can resolve, and it therefore appears like a point of light when viewed without optical aid. In order to see it as a disk, the image needs to be magnified. If viewed with 7-power binoculars (e.g. 7x50), the disk appears to be 7x40"=280" (or 4.7') wide, rendering it visible.

The aperture of a telescope (the diameter of the objective lens or mirror) determines the smallest detail that can be seen (if magnified to above the eye's 60" limit). A 60mm aperture typically has a theoretical resolution limit of 2.3" while a 25cm (10-inch) can resolve 0.56".

Dark adaptation and averted vision

One basic technique that observers soon learn is to allow the eyes 20 minutes or more to adjust to darkness in order to become sensitive to faint light (dark adaptation). Bright light spoils this sensitivity. A much-dimmed torch (shielded with red cellophane) will provide sufficient light, once dark-adapted, to read star charts and make notes and sketches. Equally important is the technique of using averted vision. Looking slightly to the side of a faint object, instead of directly at it, gives a much improved view, because the retina is most light-sensitive around the edges. It is also important to move the eyes around somewhat while observing, because an image kept at the same point on the retina is ignored by the brain after a time.

Limiting magnitude

The brightness of the faintest star that can be seen on a given night is called the limiting magnitude. A number of factors influence this limit, the three most important being aperture, dark skies, and experience.

The larger the light-collecting area of an optical system is (its aperture), the fainter the stars that can be seen. The dark-adapted naked eye has an aperture about 7mm in diameter (the pupil). Stars down to about mag. 6 (or 7 at really dark sites) can be seen. Binoculars allow more light to be gathered (a 10x50 binocular has an aperture of 50mm) and thus show fainter stars (down to about mag. 10). A telescope allows even fainter stars to be seen.

The brightness of the sky--a measure of the amount of light pollution--also influences the limiting magnitude. Viewing from a dark rural area enables one to see fainter stars than from a light polluted city.

Observing faint objects is also a skill--the eye-brain system benefits from practice and experience. A skilled observer under dark skies can see as much as two magnitudes fainter than a novice under the same conditions.

The star chart above can be used for checking the limiting magnitude around the South celestial pole with the naked eye. Table 19 summarizes how changing limiting magnitudes alter the appearance of the night sky.

Time

The measurement of time is by no means a trivial exercise as the motion of the Earth in its path around the Sun is far from uniform. The Earth's orbit is not circular but elliptical, which means, amongst other things, that the speed of the Earth around the Sun varies. In addition the axis of the Earth's rotation is inclined to the orbital plane. A brief summary of some of the terms in common use with some simple explanations follows.

Relation between Time and Angle

The Earth rotates once a day, which means that it turns through 360° every 24 hours. So one hour of time is equivalent to 15° or in other words, it takes 4 minutes to turn through 1°

Mean Solar Time

For practical purposes it is assumed that the time taken for the Sun to return to its highest point in the northern sky (crossing the meridian) on consecutive days is 24 hours. For measuring the time of day it is convenient to start and end at midnight. The first 12-hr period is often called ante-meridian or AM and the second, post-meridian or PM, commonly known simply as morning and afternoon. Today it is common to use a 24-hr timekeeping method so that 9:30 AM is 09h30 and 6:56 PM would be 18h56. Mean Solar Time, MST, is the time that is used for our daily living and assumes that the Earth moves at a uniform rate around the Sun: a sort of averaged out, or 'mean' rate. It is sometimes referred to as "watch time", being the time shown by our watches and clocks. It is officially called South African Standard Time, SAST (see below). The difference between MST and solar time is known as the Equation of Time, which also explains why one needs to make corrections to sundial time to give SAST.

Sidereal Time

From noon on one day to noon on the following day the Earth rotates a little more than 360° because, in the course of the day, the Earth has moved along its orbit a little. As the diagram shows this amounts to about 1°, since in a year the Earth turns through 360° in 365.25 days or (360/365.25)°. So a star on the meridian at midday on a particular day will be visible at midnight six months later (of course we cannot see them at midday, but they are there!).

Sidereal time agrees with solar time on about September 23 each year and from then on gains about 4 minutes (3m 56s) each day on solar time. In a month this accumulates to about 2 hours and so 24 hours in a year. The difference between sidereal time and solar time also explains why a particular star or constellation rises about 4 minutes earlier each night or about 2 hours earlier each month. Thus sidereal time is a measure of the rotation of the Earth with respect to the stars, rather than the Sun. It is the time used by astronomers to set the coordinates of their telescopes. See Table 20 on page 89.

Universal Time

In the past all world times were based on the 0° meridian (longitude) through Greenwich, London. This meant that all times were calculated from that meridian, or Greenwich Mean Time, GMT. Since January 01, 1972 this has become Coordinated Universal Time, or UTC, and is based on international atomic time, accurate to about 1 second in 1 400 000 years. In the past 1 second was defined as 1/86 400 of a mean solar day, today it is defined as 9 192 631 770 vibrations of a Caesium atom. In order to keep clocks properly aligned with the Earth's seasons, leap seconds used to be added/subtracted at the end of June and/or December. This has been discontinued and these are now added when needed. Because local time is the same as the differences of longitude from Greenwich, local mean time becomes later with increasing distances east and earlier with increasing distances west, so local time zones are still calculated in the same way: add 1 hour for every 15° east and subtract an hour for every 15° west.

The times of most events are given in what is usually called UT (strictly UTC) and to get the local time for that event one simply adds or subtracts the correct number of hours according to the local time zone. South Africa's time meridian is 30° east, through Pietermaritzburg (PIVIB), and so SAST is two hours ahead of UTC; i.e. if it is 09:43 UT it will be 11:43 SAST. South African Standard Time is used throughout the SkyGuide.

Large countries have several time zones (Australia has three from +8 to +10 hrs), South Africa has one. The famous "Noon day gun" is 'fired' electronically from the SAAO at exactly 12:00 SAST each day. However Cape Town is only 18° east and is thus 12° west of the time meridian. This means that'noon' in Cape Town is not 12:00 but 12/15 of 1 hour, or 46 minutes, later, i.e. at 12:46. The Sun crosses the meridian after 12:00 in nearly all places within SA since most lie west of PIVIB and this is why sunrise/set and moonrise/set times differ from place to place in South Africa, depending on how far east or west of the time meridian they are positioned. For details see the monthly rise and set times for selected centres in SA in this handbook. (Diary pages 2-49)

To calculate the approximate local time for any other place simply find out how many degrees west/east you are from 30° east. Work out what fraction this is of 15 and multiply it by 60 minutes and add/subtract this to/from SAST. For example, if you are 7° west of 30° east, take 7/15 of 60 minutes to get 28 minutes and subtract this from the SAST to get your local time.

When a telescope equipped with setting circles is aimed at the meridian, its RA circle should read the sidereal time. Thus one can calculate the sidereal time (Table 20) and then set the circle, but it is usually simpler to aim the telescope at a star of known RA and then adjust the circle. Table 16 on p79 lists bright stars that can be used for this purpose.

Julian Day Number

Because it is difficult to find out the number of days that have elapsed between two dates that are far apart (leap years, etc., make it a tedious process) astronomers have adopted something called the Julian Day number, JD number, or simply JD. John Herschel suggested noon on January 1, 4713 BCE, this being the time when three ancient recorded cycles first coincided. Noon was chosen since that meant that all astronomical observations would occur during the course of a single night, rather than being split by midnight.

The hours, minutes and seconds of Julian day numbers are given as a decimal for ease of subtraction, and these decimals are:

0.1 = 2.4 hours or 144 minutes or 8640 seconds

0.01 = 0.24 hours or 14.4 minutes or 864 seconds

0.001 = 0.024 hours or 1.44 minutes or 86.4 seconds

0.0001 = 0.0024 hours or 0.144 minutes or 8.64 seconds

0.00001 = 0.00024 hours or 0.0144 minutes or 0.864 seconds

The present JD number is thus the total number of days that have elapsed since noon on 1 January, 4713 BCE. Noon on 1 January 2010 will be JD 245 5198.00. The integral part gives the Julian day number and the fractional part gives the time of day since noon UT, i.e., 0.5 represents midnight UT: this means that 2010 starts at JD 245 5197.50, being half-a-day before noon on the 1 January. JD numbers are given in this handbook for 14:00 SAST on the 1st of each month. Almost 2.5 million Julian days have elapsed since the initial epoch. JD 2 400 000 was November 16, 1858 and JD 2 500 000.0 will occur on August 31, 2132 at noon UT.

Twilight

There is no legal definition of twilight, since the amount of light available to do things, such as drive or work outside, depends on factors other than how far the Sun is below the horizon, weather, buildings and vegetation being obvious ones. The following are generally accepted as guidelines for the three different twilights. The times given will vary considerably with both the observer's latitude and the progress of the seasons:

* Civil Twilight is from when the Sun sets till it is too dark to do manual work outside without the aid of artificial light. This is usually when the Sun is 6° below the horizon, or 24 minutes after sunset.

* Nautical Twilight is from the end of civil twilight to the time that the horizon is no longer visible, usually when the Sun is 12° below the horizon, or 48 minutes after sunset.

* Astronomical Twilight is from the end of nautical twilight until the Sun no longer has any effect on a clear night sky, and is usually when the Sun is 18° below the horizon, or 72 minutes after sunset.

Astrophotography

It is possible to take pleasing photographs of the night sky without elaborate equipment. Many charming celestial images may be captured using either digital or conventional film cameras. The ASSA Imaging Section was formed in 1999 to encourage and assist members to further their skills. For the latest information, projects and activities of the Imaging Section, please visit [http://www.saao.ac.za/assa/sections/imaging.html].

Imaging Section

Members interested in Astrophotography and digital imaging are encouraged to contact the Section Director. Novice and experienced observers are welcome to submit their efforts. Contact: Oleg Toumilovitch PO Box 2137 Pinegowrie 2123 Tel: 011-787-4375 Cell: 082-680-4700 e-mail: [imaging@assa.saao.ac.za]

Naked eye limiting magnitude--South Celestial Pole

This star chart, showing stars down to about 7th magnitude, depicts a 30° filed centred on the South Celestial Pole (marked with an 'x'). The star nearest the Pole is 5.5 mag s Octantis, labelled 'n'. Visual magnitudes, calculated from data in the Tycho-2 catalogue, are given for labelled stars, listed below. The capital letters around the edge of the chart are an aid to orienting it; see page 92 for instructions.

Visual magnitudes for stars around the South Pole

a 2.8

b 3.8

c 3.9

d 4.1

e 4.2

f 4.3

g 4.4

h 4.9

j 5.1

k 5.3

m 5.4

n 5.5

p 5.6

q 5.8

r 6.0

s 6.1

t 6.2

u 6.3

v 6.4

Table 18. Popular binocular configurations

7x35 Advantages: Easily hand-held; excellent wide-field
 views of Milky Way & deep-sky objects.
 Disadvantages: Smaller exit-pupil restricts dark sky
 effectiveness.

7x50 Advantages: Easily hand-held, light-gathering ability
 sufficient for hundreds of objects; best choice.
 Disadvantages: Larger aperture may cause skyglow
 problems in urban and sub-urban areas.

10x50 Advantages: Good choice for urban and sub-urban
 users who want higher magnification.
 Disadvantages: May require a tripod.

10x70 Advantages: Excellent for clusters, nebulae and
 galaxies.
 Disadvantages: Heavy; tripod usually needed.

11x80 Advantages: Excellent for faint objects.
 Disadvantages: Tripod needed; heavy.

From Touring the Universe Through Binoculars by Phil
Harrington.

Table 19. Light pollution and the night sky

Mag. Number Description
limit of stars

2.5 93 In Crux, only three stars can be seen.

3.5 283 Naked eye limit from the inner city; planets,
 Crux and Orion can be seen.

4.5 893 Naked eye limit from within cities. Night sky is
 grey or orangish. Objects on the ground are
 very clearly visible by reflected skylight. Many
 constellations unrecognizable because many
 stars are hidden.

5.0 ~1600 Naked eye limit in average semi-dark skies.

5.5 2822 Naked eye limit in the suburbs; best semi-dark
 skies. Milky Way somewhat hard to see.
 Andromeda Galaxy and the Beehive Cluster
 (M44) visible to the naked eye but not prominent.
 Clouds are bright, illuminated from below.

6.5 8768 Naked eye limit from rural/dark suburban skies.
 Some domes of light pollution on the horizon.
 Milky Way clearly visible, brightest parts are
 prominent. Good for serious observing and
 astrophotography.

7.50 26533 Binocular limit from within cities. Naked eye
 limit, truly dark skies. Milky Way elaborately
 structured. Zodiacal light prominent along
 ecliptic in the west after the end of twilight.

10.0 ~360000 Binocular limit in the best semi-dark skies.

10.5 ~600000 Limit of 15-cm telescope in inner city; 10x50
 binoculars in average, very-dark skies.

11.5 ~1600000 Typical limit of 60-mm telescope

12.5 ~4000000 15-cm telescope from best semi-dark skies.

13.5 ~10000000 15-cm telescope from best, very-dark skies,
 25-cm from average semi-dark skies.

14.5 ~22000000 25-cm telescope from best semi-dark skies;
 40-cm telescope from average semi-dark skies.

Table 20. Sidereal time for 30° longitude

Date sidereal time Date sidereal time
2010 00h 00m 21h 00m 2010 00h 00m 21h 00m

Jan 01 06h 43m 03h 44m Jul 10 19h 12m 16h 13
 11 07 22 04 23 20 19 51 16 53
 21 08 01 05 03 30 20 31 17 32
 31 08 41 05 42 Aug 09 21 10 18 11

Feb 10 09 20 06 22 19 21 49 18 51
 20 10 00 07 01 29 22 29 19 30

Mar 02 10 39 07 41 Sep 08 23 08 20 10
 12 11 19 08 20 18 23 48 20 49
 22 11 58 08 59 28 00 27 21 29

Apr 01 12 37 09 39 Oct 08 01 07 22 08
 11 13 17 10 18 18 01 46 22 47
 21 13 56 10 58 28 02 25 23 27

May 01 14 36 11 37 Nov 07 03 05 00 06
 11 15 15 12 17 17 03 44 00 46
 21 15 55 12 56 27 04 24 01 25
 31 16 34 13 35 Dec 07 05 03 02 05

Jun 10 17 13 14 15 17 05 43 02 44
 20 17 53 14 54 27 06 22 03 23
 30 18 32 15 34 31 06 38 03 39

The table lists sidereal times at longitude 30° for 00:00
and 21:00 SAST. To find the sidereal time at other locations,
correct the listed times as follows: for Bloemfontein subtract 15
minutes, Bulawayo -6m, Cape Town -4m, Durban +4m, East
London -8m, Grahamstown -14m, Johannesburg -8m,
Kimberley -21m, Port Elizabeth -18m, Pretoria -7m,
Harare +4m and Windhoek-52m.
COPYRIGHT 2010 Astronomical Society of Southern Africa
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2021 Gale, Cengage Learning. All rights reserved.

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Publication:Sky Guide Africa South
Date:Jan 1, 2010
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