Why $wp_query->posts[0]->ID and get_the_ID returns a different ID?

Question

Basically I am running those two outside the WordPress loop:

$wp_query->posts[0]->ID
get_the_ID

The first one returns of course the ID of the first post. The second one returns the ID of either the first post or the last one. I would love to know why.

Basically I need to know information about the first post when I am in the header, which is why I was trying to use get_the_ID . I cannot enter the loop at this stage.

Any help will be highly appreciated.

Answer 1

global $post;
echo get_the_id($post->ID);