What’s in the sky?

All About Space - - Contents -

The nights are get­ting longer, so dust off your kit for some great nights of ob­ser­va­tion

Jar­gon buster

Con­junc­tion A con­junc­tion is an align­ment of ob­jects at the same ce­les­tial lon­gi­tude. The con­junc­tion of the Moon and the plan­ets is de­ter­mined with ref­er­ence to the Sun. A planet is in con­junc­tion with the Sun when it and Earth are aligned on op­po­site sides of the Sun. Right As­cen­sion (RA)

Right As­cen­sion is to the sky what lon­gi­tude is to the sur­face of the Earth, cor­re­spond­ing to east and west di­rec­tions. It is mea­sured in hours, min­utes and sec­onds since, as the Earth ro­tates on its axis, we see dif­fer­ent parts of the sky through­out the night.

Dec­li­na­tion (Dec)

This tells you how high an ob­ject will rise in the sky. Like Earth’s lat­i­tude, Dec mea­sures north and south. It’s mea­sured in de­grees, ar­cmin­utes and arc­sec­onds. There are 60 arc­sec­onds in an ar­cminute and there are 60 ar­cmin­utes in a de­gree.


An ob­ject’s mag­ni­tude tells you how bright it ap­pears from Earth. In as­tron­omy, mag­ni­tudes are rep­re­sented on a num­bered scale. The lower the num­ber, the brighter the ob­ject. So, a mag­ni­tude of -1 is brighter than an ob­ject with a mag­ni­tude of +2.


When a ce­les­tial body is in line with the Earth and

Sun. Dur­ing op­po­si­tion, an ob­ject is vis­i­ble for the whole night, ris­ing at sun­set and set­ting at sun­rise. At this point in its or­bit, the ce­les­tial ob­ject is clos­est to Earth, mak­ing it ap­pear big­ger and brighter. Great­est elon­ga­tion

When the in­ner plan­ets, Mer­cury and Venus, are at their max­i­mum dis­tance from the Sun. Dur­ing great­est elon­ga­tion, the in­ner plan­ets can be ob­served as evening stars at great­est eastern elon­ga­tions and as morn­ing stars dur­ing west­ern elon­ga­tions.

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