Asteroid 5 Astraea was discovered on this day in 1845 by Karl Ludwig Henke, and, as it was still relatively unusual to find asteroids at that time, it won him an annual pension of 1200 Marks from the king. Astraea is S-type, in the main belt, is quite big (roughly 167 km across at its widest point) and takes just over 4 years to orbit the Sun.
In Greek mythology, Astraea is the celestial virgin, and her name means just that (“star maiden”). She was a personification of justice, innocence and purity, and was the last of the immortals to live down here among us humans. Upon leaving Earth she became the constellation Virgo.
Yesterday we met the concept of the semi-major axis, and if you were paying attention you will know what I mean when I say that Astraea’s is 2.57 AU. Today I’m attacking you with another position-related phrase, the longitude of the ascending node. Like yesterday’s, it sounds more complicated than it is. But unlike yesterday’s, it needs a really long rambling explanation, so here goes.
It’s an unwieldy phrase, isn’t it? So I’ll break it down into small parts. Firstly, if I tell you that for Astraea it is 141.7°, you’ll already know that it’s some sort of direction. Longitude, as you know, is a coordinate showing the east-west position of something. It tells you how many degrees you have to turn in an anti-clockwise direction from some point of reference in order to end up pointing at the place you want to be. For example, your directions might tell you that if you’re standing in the car park looking at the church (point of reference) you need to turn 141.7° to see the pub (or just follow the smell of the beer).
Next, a node is the point at which an inclined (i.e. not horizontal) orbit crosses the horizontal plane on which your reference point lies. For objects in the solar system we use the ecliptic(the plane marked out by the Sun’s apparent path across the sky) as the plane, and the First Point of Aries (ask me later – just accept it as a direction in the sky) as the point of reference. There will be two occasions in each orbit when a body such as Astraea crosses this plane: one when it goes down below it, and another when it comes back above it. This second one is the ascending node.
So, the longitude of the ascending node is the direction you would need to be looking in order to see Astraea, if you knew precisely when it was about to start heading upwards above the ecliptic. Does that make sense?
A poem from Emerson to finish (“cap-a-pie” means from head to foot):
Each the herald is who wrote
His rank, and quartered his own coat.
There is no king nor sovereign state
That can fix a hero’s rate;
Each to all is vulnerable,
Until he write, where all eyes rest,
Slave or master on his breast,
I saw men go up and down,
In the country and the town,
With this tablet on their neck,—
‘Judgement and a judge we seek.’
Not to monarchs they repair,
Nor to learned jurist’s chair;
But they hurry to their peers,
To their kinsfolk and their dears;
Louder than with speech they pray,—
‘What am I? companion, say.’
And the friend not hesitates
To assign just place and mates;
Answers not in word or letter,
Yet is understood the better;
Each to each a looking-glass,
Reflects his figure that doth pass.
Every wayfarer he meets
What himself declared repeats,
What himself confessed records,
Sentences him in his words;
The form is his own corporal form,
And his thought the penal worm.
Yet shine forever virgin minds,
Loved by stars and the purest winds,
Which, o’er passion throned sedate,
Have not hazarded their state;
Disconcert the searching spy,
Rendering to a curious eye
The durance of a granite ledge
To those who gaze from the sea’s edge.
It is there for benefit;
It is there for purging light;
There for purifying storms;
And its depths reflect all forms;
It cannot parley with the mean,—
Pure by impure is not seen.
For there’s no sequestered grot,
Lone mountain tarn, or isle forgot,
But Justice, journeying in the sphere,
Daily stoops to harbour there.
Ralph Waldo Emerson (1847)
1842 ⇒ Birth of Alphonse Louis Nicolas Borrelly, the French astronomer who crops up in these pages every few weeks as a discoverer of asteroids. He found 18 in all, and discovered or co-discovered six comets.