BBC Sky at Night - UK (2021-08)

Pete Lawrence is an expert

astro imager and a co-presenter

on The Sky at Night on BBC TV

ALL

PIC

TUR

ES:

PE

TE^

LAW

RE

NC

E

August 2021 BBC Sky at Night Magazine 71

to determine how much the feature is drifting within

the planet’s atmosphere. By plotting time versus

longitude you can generate a drift chart to tell you a

great deal about how the planet’s atmosphere works.

Features with tangible widths can be measured

using the planet’s rotation. Imagine the leading

(preceding) edge of a feature reaching the CM at time

Tp. The feature’s trailing (following) edge will pass the

CM at time Tf. The feature’s width can be determined

by converting these to longitudes Lp and Lf using

WinJupos (http://jupos.org). The physical width in

kilometres is calculated with the following formula:

(Lf–Lp) x 1217.4 x cos(latitude)

All timings should be done in Universal Time and it’s

important to use the correct longitude System, I or II.

This brings us to Jupiter’s Great Red Spot (System II),

an extended feature (see box, opposite). CM timings of

its preceding and following edges, along with timings

that record its centre crossing the CM, give valuable

information. While edge timings give its width, the CM

timing reveals its central longitude. Over the years the

GRS’s physical width changes and its longitude drifts.

,I\RXFDQPDVWHUWKHVHXVHIXOVFLHQWLƅFREVHUYLQJ

techniques, it will help to enrich your planetary

observing for many years to come.

Using a telescope and your eyes,

intensity estimates can be made of

Saturn. You’ll need a planetary

blank to show the correct outline of

the planet for the observing date.

WinJupos can generate blanks by

selecting Saturn from the ‘Program’

> ‘Celestial body...’ option, before

choosing ‘Tools > Ephemerides...’ and

going to the ‘Graphics’ tab. Make sure

the date is set correctly and turn off

any textures, etc, to show a planetary

outline which can be printed.

At the eyepiece, the intensity

estimate is done by dividing the

planet into areas of equal brightness.

A numeric scale of 0 to 10 is then

used to annotate each area

according to its brightness, 0 being

bright, 10 dark. Intermediate,

decimal values may be used if

required. In order to calibrate any

estimates, two fixed areas are used.

The outer part of ring B, the brightest

of Saturn’s rings, is given an intensity

of 1. The dark shadow of the planet’s

globe on the rings is given a value

of 10. In the case of the brightness

value, brighter areas may be seen

and these should be given decimal

values less than one, up to a

maximum of 0 for an exceptionally

bright region.

Observer bias can be eliminated

by first labelling regions brightest

to darkest. Once complete, start on

a new blank but label darkest to

brightest. Although tricky at first,

it doesn’t take long before making

accurate estimates becomes second

nature. The technique is also a great

way to connect to Saturn visually,

forcing you to look at every part of

the planet.

How to make accurate visual brightness evaluations in different sections of Saturn

Making intensity estimates

Þ WinJupos can be used to determine longitudes of the times (T1, top and T2,

bottom) when the edges of the Great Red Spot touch the Central Meridian

Annotate each area of Saturn according to brightness

by numbering them on a scale from 0 to 10

S

>