Las efemérides indican cuándo observar el objeto celeste.

Because efeméride is a feminine noun in Spanish. Its plural is efemérides, so it takes the feminine plural article las.

• singular: la efeméride
• plural: las efemérides

Even though the ending -es may not look obviously feminine to an English speaker, the gender is still feminine.

In this context, efemérides refers to tables or data that show the positions of astronomical objects at different times. In astronomy, this is a technical term, and English has a very similar word: ephemerides.

So in this sentence, las efemérides are the source of information that tells you the right time to observe something in the sky.

Because the subject is las efemérides, which is plural. In Spanish, the verb must agree with the subject in number.

• la efeméride indica = the ephemeris indicates
• las efemérides indican = the ephemerides indicate

So indican is the third person plural form of indicar.

Cuándo has a written accent because it is being used in an indirect question: when to observe.

Spanish uses accent marks on words like qué, cuál, cuándo, cómo, dónde when they introduce a question, whether direct or indirect.

Examples:

• ¿Cuándo vienes? = When are you coming?
• No sé cuándo vienes. = I don’t know when you’re coming.

In your sentence, cuándo observar el objeto celeste means when to observe the celestial object, so the accent is required.

Cuándo observar is a very natural structure in Spanish for when to observe + object.

It uses:

• cuándo = when
• observar = to observe

This is similar to English infinitive structures like:

• where to go
• what to say
• when to leave

Here, the object is stated separately as el objeto celeste, so there is no need for -lo.

You could say cuándo observarlo if the object had already been mentioned and you wanted to replace el objeto celeste with it:

• Las efemérides indican cuándo observarlo.

Because after cuándo in this kind of structure, Spanish often uses the infinitive when the subject is general or unspecified.

So:

• cuándo observar el objeto celeste = when to observe the celestial object

This is the same pattern as:

• qué hacer = what to do
• dónde comer = where to eat
• cómo llegar = how to get there

If Spanish wanted to specify a different subject, it could use a conjugated verb instead:

• Las efemérides indican cuándo debemos observar el objeto celeste.
  = The ephemerides indicate when we should observe the celestial object.

Because Spanish infinitives do not need a separate word like English to.

English says:

• to observe

Spanish simply uses the infinitive form itself:

• observar

So cuándo observar literally looks like when observe, but it means when to observe.

In Spanish, adjectives usually come after the noun, especially when they are descriptive and factual.

So:

• objeto celeste = celestial object

This is the normal word order. Putting the adjective before the noun is much less common and often gives a literary, emphatic, or stylistic effect.

For ordinary scientific language, el objeto celeste is the most natural order.

Both can relate to the sky or heavens, but they are not always used in exactly the same way.

• celeste often means heavenly, celestial, or sometimes sky-blue depending on context.
• celestial is also a valid adjective meaning celestial, and it can sound a bit more formal or literary in some contexts.

In astronomy, objeto celeste is a very common and natural expression in Spanish for celestial object.

Yes. Indicar often means to indicate, but depending on context it can also be translated as show, tell, or point out.

Here, Las efemérides indican cuándo observar el objeto celeste could naturally be understood as:

• the ephemerides indicate when to observe the celestial object
• the ephemerides show when to observe the celestial object

The exact English choice depends on style, but indicate is the most direct match.

Yes, observar is a good choice here because it suggests careful or deliberate observation, which fits an astronomical context very well.

Compared with similar verbs:

• ver = to see
• mirar = to look at
• observar = to observe, watch carefully

For science or astronomy, observar sounds more precise and appropriate than mirar.

In standard Peninsular Spanish, a rough pronunciation would be:

las efeMÉrides indican CUÁNdo obserBAR el obJÉto ceLÉSte

A few useful notes:

• c before e in celeste is pronounced like th in many parts of Spain: the-LES-te
• v and b sound very similar in Spanish
• the stress falls on:
  • efemérides
  • indican
  • cuándo
  • observar
  • objeto
  • celeste

Yes, but the original order is the most neutral and natural.

Original:

• Las efemérides indican cuándo observar el objeto celeste.

You could also hear variations like:

• Las efemérides indican cuándo se debe observar el objeto celeste.
• Las efemérides indican el momento en que debe observarse el objeto celeste.

These are grammatically correct, but they sound more formal or more explicit. The original sentence is concise and natural, especially in technical writing.